Shivalinga is also known as shivling or shiva linga. Shivalinga is representation of lord shiva. Shivalinga is used to worship shiva and make him happy. Lord shiva is worshipped in form shivalinga and human image form (either any metals or things or pictures).
Shivalinga comes from two words (shiva+linga). Shiva represent lord shiva and linga represent (symbol or representation).
It is said that at bottom of shivalinga there is lord brahma(creator of universe),vishnu in middle(preserver) and shiva at top(destroyer of ignorance).
Shivalinga also symbolize mother parvati and shiva (husband and wife). Lord shiva as part and mother parvati as tail part. Mother parvati got shiva husband by worshipping shivalinga.
Shivalinga fullfill the wish,cleanses the sin, pleases shiva parvati,brahma vishnu and all god.
There is no god like shiva, so he is known as mahadev (great or big god).