Digital collections are an organized and described (using metadata) group of media objects. Objects can be digitized or “born digital,” media of various types, and are usually searchable and/or browsable. Digital collections can be included in digital exhibit sites--platforms like Omeka lend themselves to this as they allow for the creation of both.
Metadata is information about different types of data, which includes media objects. Some types of metadata are descriptive, which explains characteristics of data/media objects, administrative, which explains things like resource type and copyright, and structural, which explains things like the version of the data/media object and the relationships between different objects within the collection.
Digitization is the act of making a digital version of analog media. It involves the scanning of a physical photograph and transferring of an analog audio tape to a digital audio format, for example.
Born digital refers to media that is digital in its original form, e.g., a Word doc, a photograph taken with a digital camera, and a Photoshop file.