What is the use of UPDATE statement in SQL? How is it different from ALTER statement?

UPDATE statement is used for modifying the existing records in a table. The UPDATE command specifies the rows to be changed using the WHERE clause and the new data using the SET keyword.
Syntax:
UPDATE <table name>
SET <fieldname> = <value or expression>
WHERE <criteria>

UPDATE	ALTER
It modifies the existing data of a database	It modifies existing structure of database
It is DML	It is DDL
It can only modify one or more records in a table	It can add, delete or modify column of a table.