What is ladder logic and why is it used in PLCs?

Ladder logic is a graphical programming language resembling electrical relay circuits. Programs read from left to right, top to bottom, with rails on each side representing power. Contacts (normally open/closed) represent inputs, and coils represent outputs. Ladder logic is popular because electricians familiar with relay logic can easily understand it, it provides visual troubleshooting, and it directly maps to physical I/O. It remains the most common PLC programming language in manufacturing.