The difference between INSERT, UPDATE, and MODIFY

Here's a very common syntactic question: What's the difference between INSERT, UPDATE, and MODIFY?
The answer is plain and simple:

INSERT - inserts a new record. INSERT expects that a record with the required key does NOT exist in the table. Therefore, if a record with the same key already exists, a runtime error occurs. For more details on INSERT, visit here.

UPDATE - update an existing record. UPDATE expects that a record with required key exists in the table. If a record with the required key does not exist, it issue an error (sy-subrc is 4). For more details on UPDATE, visit here.

MODIFY - acts like a combination of INSERT and UPDATE. If a record with the specified key does not exist, it adds it to the table. If a record with the specified key does exist, it modifies it. So MODIFY actually acts like "Insert or change record". To read more about MODIFY, visit here.

To summarize:
INSERT - adds a new record to the table. If the row (key) exists, issues an error.
UPDATE - updates an existing record to the table. If the row (key) does not exist, issues an error.
MODIFY - If the key exists, modifies the record. If the key does not exist, adds the record to the table.