ASIC - application specific integrated circuit - has very wide definition. If somebody is talking with you about ASIC you must ask him about definition first. Otherwise you may be confused. Sawaak mentioned about one of the possible way to design and manufacture ASIC device - using Gate Array approach. Another way - Standard Cell approach - will give you a smaller chip size, but you have to buy full mask set, not only metallization masks. Third way - full custom layout - will provide the smallest chip size and best performance but designing device using this way will take much more time and money.