Learning Objects are the hot topic of e-learning (once distance learning) discussions.
You can find dozens of "official" definitions of learning objects, we recap here what is generally called a learning object:

• an instructional resource in digital format
• a reusable resource combined with other related resource
• it is self-explaining about the scope for which it was developed
  

LMS evolution brings new value to contents and their modularity. Learning Objects play a significant role in these environment only if they are independent from the platform they are installed.
That's why it is important to develop under accepted standard.

One of the most appreciated theorist is David Wiley, his work is published under the terms of the Open Publication Licence.
This link will take you to his book "The Instructional Use of Learning Objects".

On this side of the ocean, the European Community has a site about
e-learning that can be viewed here.