Knives are divided into the 4 categories above as follows:
Fixed - Knives that are as they are. Most of them come with sheaths.
Folders - Blades that can fold into their handle. Some models have 2 or even 3 blades.
Multi-Tool - Most pliers-designed tools (Leathermans) but some other models as well
Knife Related - Knife maintenance (eg. oil, sharpeners), saws and scissors.