There are at least two types of .NET attribute. The first type I will refer to as a metadata attribute - it allows some data to be attached to a class or method. This data becomes part of the metadata for the class, and (like other class metadata) can be accessed via reflection. An example of a metadata attribute is [serializable], which can be attached to a class and means that instances of the class can be serialized.

[serializable] public class CTest {}
The other type of attribute is a context attribute. Context attributes use a similar syntax to metadata attributes but they are fundamentally different. Context attributes provide an interception mechanism whereby instance activation and method calls can be pre- and/or post-processed. If you have encountered Keith Brown's universal delegator you'll be familiar with this idea.