Assembly is unit of deployment like EXE or a DLL.
When you create a code and build the solution, then the .NET Framework convert your code into Intermediate Language and that is placed inside the assembly(dll), which you can find inside bin folder.
There are 3 types of assemblies:
Private assembly - A private assembly can be used by a single application only. It is not accessible outside. So, all the projects you create will by default create private assembly only.
Public/ shared assembly - Shared assemblies are usually libraries of code, which multiple applications can use. It is registered in the global assembly cache(GAC).
Satellite assembly - A satellite Assembly is defined as an assembly with resources only, no executable code.