Where stomp boxes do one thing well, rack units tend to do either one thing very well or many things. A rack is an expensive solution, but many big-name guitarists use them - either in combination with pedals or alone. Each unit in a rack will have inputs and outputs like a pedal. The gutiar input is routed to the rack and each unit is patched together like your stomp boxes, then sent to the amp (or amps). In general, stomp boxes allow you to customize your setup as you like for a reasonable price, while keeping the programming simple. Rack units offer maximum flexibility, are expensive, and generally require much more programming to get the sounds you want.