shiva lingam is the holy symbol of Lord shiva that is considered quite sacred and worshipped with devotion by the devotees. The sanskrit word lingam means symbol, so shiva lingam means symbol of shiva. It is considered the foremost sacred symbol for shaivaites and has been worshiped for ages. The purANas like shiva mahA purANam discuss about the superiority of linga worship.
Shivlinga stands for the union of Shiva / consciousness with Shakti/ energy and that is the cause of the cosmos.
Three main Lingas we have.
First is material form which usually call Linga. Shiva, Kriya, and Ishta is older names for this outside or material form. We have and few parts of one very holy Linga in Kaaba which have and next two deeper forms)! Today have many cults which haven't real knowledge about inner form and way for enter in process bhakti, jnana, dhyana - and have only parts of this knowledge because usually make only puja.
Inside each regular Kriya/Ishta linga is Shiva linga. Kriya/Ishta Linga present form of awaking consciousness which is feeling consciousness (bhakti, pure love, first devotional state, dominant kriya shakti - any action God is love) and have and deeper level consciousness inside as willing consciousness (dominant icha shakti, love have exact real direction, or life, quality, quantity, intention,..).
Next and inner form of linga is Pranalinga. This is Linga from inner light. Saints from old time teach people about form Linga because this great souls see "Divine soul in Linga". Divine Soul has divine aura which is exact form linga. Position of Soul which old saint see (in aura) is form Shiva Linga. Now we have two forms in one. Outside is like sharp ball on the water (sphere which haven't part in down), and inside is form body in position watching Linga on the left palm. Shivalinga is symbolic form: part sphere form is only symbolic form of head (haven't nothing with phallus), middle form of Shivalinga is symbolic form of left palm and higher part of body from heart/navel to neck (haven't nothing with vulva) and on palm is subtle/invisible Linga, lower part of Shivalinga is symbolic form of lower part of body from navel to legs.
Pranalinga present form of sleeping consciousness which is cognition consciousness (third devotional or bhakti state, where is dominant jnana shakti - any direct knowing is God love) and have and deeper level consciousness inside as intuition consciousness. This is point reflection consciousness on the lower states or enter in higher form (execution) consciousness which is only base reflection of intuition consciousness.
Deepest form Linga is Mahalinga. This is very high dynamic form, with very high speed rotations and quietness (of extremely higher forms inner lights and inner sounds which is incorporate simultaneously), and all this is complete beyond mind. Very difficult find any reasonable words for intellect. Much deepest and complexes forms than in illusion when we look car which go in one direction, but all time wheels go on opposite direction. Here all forms have source or there is real place for resolve all illusions and then direct see God (or enter in complete consciousness). This extreme rotations have outside form Linga (sphere without lower part).
There where all (para, chit, ananda, jnana, icha and kriya) shaktis deepest with Shiva, became completed in One Consciousness or enter in reflection as lower consciousness.
This consciousness can abide as one consciousness on lower level as Pranalinga or as consciousness which reflected as thought in mind and intellect (or became omitted, or limited consciousness) which present as all outside forms of pranalinga.
Mahalinga present form of deep sleeping consciousness which is execution consciousness (fifth devotional or bhakti state) and have and deeper level consciousness inside as union consciousness (sixth devotional or bhakti state) when haven't separation in consciousness.
Because each of this three Lingas have two Lingas, exactly we have 6 Lingas*** or 6 levels consciousness or consciousness and five reflections. Now is open: not any creation all is only reflection, all is one consciousness, or all is One!
Real teachings is only part of this! This is and real meaning in doctrines hexagram, pentagram, and six days creation,..
Shape of outside Linga is like shape inside Lingas which is from lights. This Linga's shape is open only through inner experiences. When real saint call God, or better say make proper invocation to sanctify shape Linga (which made in one stone), and when properly make sanctify one small part of consciousness enter in stone, and stone became sacred. Of course and today many try make Linga, but result is very different from this what will be necessary.
Linga is from oldest teaching, and connection with this knowledge have and spread great prophet Mohammed (and today we have only few parts one from this sacred stone in Kaaba). Have doctrines which want explain God and His creation like eggs, but haven't this crucial static part or horizontal base Linga. Haven't any part under base complete invisible and static on the all levels, and when seeker truth try to find anything deeper.Shape Linga (and through original Kaaba's sacred stone) on this way teach us: God is inside all His emanation (energy, creation) as base which give all.
Original colour Linga is white in Gold / Krita Yuga, but today is black because in last period is Kali Yuga! This black colour is originate and with colour pigment in pineal gland, which exact deep indigo colour (very close black).
Today we have many religion variations and teachings, and very probably lose many crucial points. Direct prayer to God (five prayers per day teach us God through Islam) is very simple form, but today people have many rules and variations. This is bad, through this rules nobody can't enter deeper then level which produce this rules! This is not God. When we accept deepest form which higher then any intellectual efforts, and we for this have all time direct highest God's form or Linga! This way is only intuitional way or through open heart (consciously way).
An ancient story in our scriptures:
Once the god of creation brahma and the god of protection mahA vishNu entered in an argument on deciding who is greater. When those two great gods were fighting between themselves, the Greatest of all Lord shiva who is formless and transcends everything appeared as a pillar of flame. God told brahma and vishNu that whoever finds the head or the foot of His flame form would be considered greater. brahmA took the form of swan ( ha.msam ) and set out to reach the top of the Fire. vishNu became a wild pig to see the foot of the Fire. Where are the limits for the infinite God ? They could not succeed in spite of their heavy effort. They realized their mistake and the peerless greatness of Lord shiva. The God who can never be reached by ego appears as the form of love to those who surrender to Him. Lord shiva appeared in the form of shiva lingam (which is the shape of flame) for their benefit. They worshiped that oldest form of Him and got blessed. The God who came to bless them from the Flame, from the lingam is known as li.ngodbhava (2). This incident is depicted in shiva mahA purANam (1) and many other scriptures.
As the God stood as pillar of flame, the natural adornment for the Lord is Holy Ash. (As ash would be found on the surface of the fire). As the God stood as the Luminat Flame that stood up the shiva sahasranAma hails God as Urdva retas and the shiva aShtottara as hiraNya retas. The hymn that starts nidhana pataye namaH, refers to many similarities of Flame and shivalinga.
God is formless, attributeless and omnipresent. This state is called arUpa (formless). For the benefit of the pashus, He took the form of shiva lingam which is neither form nor formless but is a symbol. This symbol of Him is called arUparUpam (formless form). He took many forms out of His mercy to bless all, which are called rUpams. (2). shaivam worships the God in all these three states. The oldest form that the formless God took namely the shiva lingam is called sadAshiva mUrtam and is worshipped in the altar of His abodes (temples). The forms that He later took which are called mAhEshwara mUrtam are worshipped in the outer circles in a temple.
This shiva linga worship is superior because it makes the worship simple because of the form while maintaining the truth that God is not having any definite form. There are a lot more marvelous principles behind the shiva linga worship which could be found in detail in standard scriptures.