Sequences are used to initiate multiple touch points. Sequence emails have the option to include and unsubscribe link within them. You can find the option to include the unsubscribe link within the Sequence settings. Sequences also log and display a lot more data around performance, and also retain that data per prospect historically.
Bulk compose emails are a single, one-time email that you can send to multiple prospects. Bulk compose emails do not contain an unsubscribe link. NOTE: if you include multiple prospects within a bulk email, each prospect will NOT be able to see the other prospects that were included in the bulk compose.