An Attribute is a declarative tag which can be used to provide information to the compiler about the behaviour of the C# elements such as classes and assemblies.
C# provides convenient technique that will handle tasks such as performing compile time operations , changing the behavior of a method at runtime or maybe even handle UN-managed code.
C# Provides many Built-in Attributes

Some Popular ones are

- Obsolete
- DllImport
- Conditional
- WebMethod