The major components of Mono include:

Code Execution Engine

Class Libraries

Base Class Library

.NET Compatibility Class Libraries

Mono specific class libraries:

Cross platform class libraries for both Mono and .NET (Gtk#, Mono.Cecil, Mono.CSharp, Text.Templating)

Unix-specific class libraries (POSIX, Filesystem in Userspace (FUSE), curses)

Platform-specific class libraries (bindings for: Mac, iOS, Android, MeeGo)

CLI Assemblies

CLI Metadata

Mono's Common Language Runtime

Compatible with the ECMA Common Language Infrastructure/.NET Common Language Runtime

Mono-specific enhancements:

Mono.SIMD support

Mono co-routines and continuations.

Mono-specific enhancements

Native interop services and COM interop

Security - Transparent Code Framework