In both MySQL 5.7 and MySQL 8, BEGIN and END is the same as in T-SQL and represents a "compound statement" also known as "a block of code", just like curly-braces in C, Java, C#, etc.

• MySQL 5.7: https://dev.mysql.com/doc/refman/5.7/en/begin-end.html
• MySQL 8.0: https://dev.mysql.com/doc/refman/8.0/en/begin-end.html

However, the BEGIN keyword is also (confusingly) overloaded as an alias for BEGIN WORK and START TRANSACTION, and their semantics depend on if they're being used inside a stored program or not:

Within all stored programs (stored procedures and functions, triggers, and events), the parser treats BEGIN [WORK] as the beginning of a BEGIN ... END block. Begin a transaction in this context with START TRANSACTION instead.

So:

• START TRANSACTION
  • Always starts a transaction. You should prefer this syntax.
• BEGIN:
  • If you're in a Stored Procedure, Function, Trigger or Event, then BEGIN by itself marks the start of a compound statement. You can only use START TRANSACTION to start a transaction.
  • If you're directly executing SQL against MySQL, then this also starts a transaction (as it's interpreted as BEGIN WORK). But it's silly and confusing to use it this way, so avoid it.
• BEGIN WORK:
  • This is an alias for START TRANSACTION. I'd avoid using this completely to prevent confusion.