A block is the square you sew. The swap is an exchange.
Sometimes the block in a swap is predetermined and sometimes just a part of the block is predetermined. For example, the Chicken block exchange expects a chicken (of sorts) and the Chocolate Raspberry swap expects a block (any block) in those colors.
For a swap, usually, a set of like blocks is sent to the organizer who then sorts the blocks and sends a random set back to every participant.
Some people make more than one set and they will get one block back for every one they submit.
In the doll quilt exchange, two quilters are partnered up and each makes a doll quilt for their partner.