What is an INDEX in MySQL and why is it important?

An INDEX is a data structure that improves query performance by creating shortcuts to data.

Benefits:
• Faster SELECT queries
• Faster WHERE, ORDER BY, GROUP BY
• Unique constraints

Types:
• PRIMARY (unique, not null)
• UNIQUE (unique values)
• INDEX (regular index)
• FULLTEXT (text searching)

Example:
CREATE INDEX idx_email ON users(email);
CREATE UNIQUE INDEX idx_username ON users(username);