The fully qualified names of .NET assemblies and types (commonly known as the 4 or 5 part name) is commonly used within the .NET world to dynamically load types from external assemblies. It is also very commonly used within the SharePoint world to denote things like feature receivers, event receivers etc etc.

An assemblies fully qualified (4 part) name consists of the following elements seperated by the , (comma) character;

1. The assembly name (without the .DLL extension)
2. The version
3. The culture
4. The assemblies public key token

MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=110f58969ae7fef2
To determine the fully qualified name of your assembly, you can can use the excellent Reflector, or this utility I knocked up.

Types within an assembly also have a fully qualified name (this is the 5 part name) and is composed of the following elements seperated by the , (comma) character;

1. The types fully qualified (by namespace) type name
2. The assembly name (without the .DLL extension)
3. The version
4. The culture
5. The assemblies public key token

As an example, suppose I have a type in the above assembly (MyAssembly) called MySampleType in a namespace called MyAssemblyUtils, the fully qualified (5 part) type name will therefore be;
MyAssemblyUtils.MySampleType, MyAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=110f58969ae7fef2
This however is a simple explanation sufficient for most cases, for much further detail on how fully qualified type names are constructed check Google or MSDN.