A particle emitter consists of an emitter and one or more particle cells. When you choose a particle emitter from the library it already is complete in that it has an emitter and particle cell with the emitter controls set to accomplish the effect you see in the preview window. When you select emitter from the tool bar you get just an emitter and you have to create and add the particle cell(s) which can be shapes, pics, drawings from drawing program etc. You then configure the particle emitter in the inspector for the effect you want such as direction of emission, lifetime of particles, point, line or shape of the emission, speed etc. In essence the particle emitters from the library are complete and if you find one that suits your need it saves a lot of work. You can, of course, adjust the effect in the inspector to your liking. The tool bar approach allows you to create your own particle emitter from scratch just as you want it. I suggest you read about emitters in the FCP user manual as it is very descriptive.