Classical architecture is the way people used to build really fancy buildings a long, long time ago. It's like building blocks, but much bigger and much older.
The way classical buildings look is very special. They have big columns that look like giant sticks holding the building up. They also have a triangular-shaped roof called a pediment. The columns can be different sizes, but usually, they have a design that makes them look similar to each other.
Many important buildings, like fancy houses or government buildings, used classical architecture in ancient Greece and Rome. A lot of people still think classical buildings look really pretty, so they still copy the designs today on buildings, like courthouses or banks.
So, if you see a building with big columns or a triangle-shaped roof, it might be classical architecture, which means it is very old and very special.