This doc provides an overview of Glazier, along with the main README:
https://github.com/google/glazier/blob/master/doc/setup/about.md
At a very high level, imaging basically always boils down to defining a series of tasks, so in a sense, the two are similar. Glazier is quite literally a tool that processes a series of tasks.
Products like MDT may provide a UI for the process. UIs are nice in that they tend to be easiest to learn and implement. In Glazier the tasks are hand written, and the source code for the entire product is freely available. This requires a bit more effort to set up and become familiar with than a GUI. The trade off is that you are not restricted to what the product UI allows you to do. You can manipulate the tasks any way you like, and even extend the product with new features.