A block is when the email server you are sending to refuses your email. There can be lots of reasons for this: they might decide you are spam, that you violate an internal policy they have, or the user you are sending to may no longer exist on the mail server.
A bounce is an email that will be sent back to you because of some error; bounces can be categorized as hard-bounces or soft-bounces. Soft-bounces are 4xx error level and are often temporary. A soft-bounce will reach the recipients email server, and then be sent back due to a full destination inbox, a server error, or an email address that no longer exists. Hard-bounces are 5xx error level and are permanent failures; hard bounces will be sent back because of an invalid address.
A drop is if it has already bounced, or if it is invalid. These occur to help protect sender reputation.