Async requires strict timing on the sending and receiving sides. Each bit has a fixed time period that both sides know. It also uses a START bit to tell the receiving side when the data stream starts.
Sync uses a clock, typically on another wire, that synchronizes the sending and receiving side timing. This way the receiving side knows when a bit is valid from the state of the clock signal.
Now go do some web searches and reading. It can help tremendously if you draw the bit pattern wave forms and send/receive data with pencil and paper. It is not that difficult once you spend the time the understand the concepts.
Then try writing code to do serial transfers by bit banging.
