A collection is a group of objects.The .NET frameworks provide generic classes that represent various types of collections, such as list, queue, set, map, and others. Using these classes, we can easily perform operations such as insert, update, delete, sort, and search on a collection of objects.

not thread-safe collection

The List collection

The Stack collection

The Queue collection

The LinkedList collection

The Dictionary collection

The HashSet collection

l thread-safe collections are reside in the System.Collections.Concurrent namespace.
System.Collections.Generic namespace can be broadly grouped into two categories:

Mutable, which support operations for changing the content of the collection such as adding new, or removing existing elements. Read-only collections, which do not provide methods for changing the content of the collection.