Can someone explain what the Compositing and Sequencer check boxes in Render Settings do? Basically I rendered a scene with several layers, then used the compositor to stitch them together. Next I saved out my layers as multi layer EXR and in the compositor replaced the Render Layers with the EXR file. At that point pressing F12 to render seemed to not do anything. I googled around and I found solutions that advised to turn off "Sequencer" ... however in my case what solved the problem was turning off "Compositing". So basically the problem is solved but it would be nice to understand how things work and what exactly happened.
Those two check boxes toggle using the compositor and sequencer.
If you are using any compositing nodes, then you will need to have the Compositing box checked.
If you are using the VSE for anything, then likewise you will need to have the Sequencer box checked.
If you are just rendering from a 3D scene, then you can uncheck both boxes.
Now most of the time you can safely leave both of them checked.
When the VSE is empty it will do nothing. However when the compositor is empty, and the Compositing box is checked, like your issue, the render will turn out blank.
See my answer for a bit more on the compositor's required nodes.
The Compositing checkbox enables the usage of the RenderLayers Compositor during a render. Likewise, the Sequencer enables the Video Sequencer Editor. This is a tool that can be used to add audio, misc. video and images, transitions, etc. to a render.
The VSE is the only totally free high quality video editor in existence. It does not have many transitions, but Blender's Python scripture and 3D Features can combine or work separately to create many custom ones.
Hope this helps and happy Blending!