A loom is anything that will hold warp threads under tension. The differences are how the sheds are achieved and how the warps are held.
You don't need a fancy loom to be able to weave and produce beautiful fabrics. The most simple of looms, in use today around the world and is probably as old as weaving itself, is the backstrap loom. The loom consists of some sticks and string.
Back Strap Basics at Weavezine.com
Laverne Waddington's Photo Stream on Flickr
Here's a fabulous picture showing a backstrap loom and the name of all its parts:
If you have a rigid heddle reed but no frame, this can be an option for you! If you don't have a rigid heddle, make one from popsicle sticks!
Below is a video I found on Youtube of a weaver using a rigid heddle on a backstrap loom.
Here is master weaver Laverne Waddington showing us backstrap basics: