A few essential concepts:
Part —A part provides services to other parts and consumes services provided by other parts. Parts in MEF can come from anywhere, from within the application or externally; from an MEF perspective, it makes no difference.
Export—An export is a service that a part provides. When a part provides an export, it is said that the part exports it. For example, a part may export a logger, or in the case of Visual Studio, an editor extension. Parts can provide multiple exports, though most parts provide a single export. Every Export has a contract, which determines what Imports it will be matched with. An export is a value (capabilities) that a part provides to other parts in the container
Import—An import is a service that a part consumes. Composable Parts declare imports through the attribute [Import]. The [Import] attribute declares something to be an import; that is, it will be filled by the composition engine when the object is composed. Every import has a contract, which determines what exports it will be matched with. An Import is a requirement (Dependencies) that a part expresses to the container, to be filled from the available exports.
Contracts—A contract is an identifier for an export or an import. The contract consists of a string, called the contract name, and the type of the exported or imported object, called the contract type. Only if both the contract name and contract type match is an export considered to fulfill a particular import. An exporter specifies a string contract that it provides, and an importer specifies the contract that it needs. MEF derives contract names from the types that are being exported and imported, so in most cases you don’t have to think about it.
Catalog – A catalog is an object that makes available parts discovered from some source. MEF provides catalogs to discover parts from a provided type, an assembly, or a directory. So basically Catalog provides the parts to the application.
Composition—Parts are composed by MEF, which instantiates them and then matches up exporters to importers. The core of the MEF composition model is the composition container, which contains all the parts available and performs composition. (That is, the matching up of imports to exports.) The most common type of composition container is CompositionContainer. In order to discover the parts available to it, the composition containers makes use of a catalog.