Start with Books Online. Every question you asked is answered in there.

A snapshot is a starting point for replication. Replication can't keep track of every transaction forever, so it needs to have a starting point. If you are initializing from a snapshot, it is your starting point. It applies the snapshot to the subscriber when you initialize it and then starts synchronizing transactionally from that point forward (from the point that the snapshot was created, not when it is appleid).

There are other ways to initialize a snapshot such as from a backup file or manually (nosync). How you initialize it originally affects how it is reinitialized. For example, if you initialize from a backup, you have to reinitialize from a backup.