Although the multitude of available rack gear may look daunting, it's really fairly simple.
Most rack mounted gear can probably be catagorized into three types:
Microphone Pre-amps, Effects Processors, and Sound Modules.
A mic pre-amp of course takes the signal from a microphone, and amplifies it to line level. This makes it possible for all the rest of the recording equipment to work with that signal.
Effects processors come in many types and flavours including, but not limited to, EQ, Reverb, Compression, delay, and a host of others.
Many effects processors are multi effect processors, and can encompass combinations of those effects listed above.
Sound modules are available for various input devices such as a keyboard, midi controller, or even a guitar, and generally offer a multitude of sounds ranging from eclectic to classical.
Operation of each of those types is well beyond the scope of this board, but in general, each of those types of rack mounted gear utilize input and output connectors to route the signal they produce to the desired location, be it a tape machine, sound card, or audio monitors.
Hope that helps a little, others will surely chime in and offer more advice/direction.
There are even rack mounted mixers available.