You can view either with Viewers or with an external Viewer Window. Viewers are also nodes you can choose from the Output window. Actually, nodes are only processed if they have an output node at the end of the node tree.
Viewer can be of double, full, half or quarter, eightth and sixteenth size (relative to original size).
Viewer windows can be of any size, and also Full Screen, if you have a second monitor.
The Viewers can be updated automatically and manually:
If you have two inputs, then you can view them in split screen. Use the wipe parameter to define the split position (50 being center).
The images are cached in RAM and as files, so when you change parameters and view back and forth, they are not recalclated again. You can set the size of the RAM and the file cache in the preferences.
Note: You can always stop an update pressing the command and the option key.