Peter was correct in stating that he could not eat these things, since the time of Moses these items were considered unclean or common. These animals are forbidden according to Jewish Laws. Peter is saying that he has never eaten anything that is forbidden. Peter was a devout Jewish man, he knew the laws and he followed them. Things that are common or are unclean are defined as defiled, unholy, impure, foul.
Abomination means disgusting or abhorrent.
God spoke to Paul and told him what was unclean is now clean because I say so. This vision was repeated three times to Peter. God wanted to make sure Paul got the message. This event is significant for several reasons. It shows the cleansing power of God.