Operations can be described as the simulation of real world car forwarding and switching. Remember that a real railroad has a purpose to exist, to haul materials from point A to point B, otherwise there would be no point in laying down miles of steel ribbon. 

In model railroads, the prototype can be simulated in switching cars, meeting schedules  and of course, yard work. Taking the time to setup a layout that can be run as both a roundy-round and an operations layout will make the best of your space. Not everyone is set out for operations though, some enjoy it, while others prefer to watch the train chase its tail around the layout. 

Basic switching can be accomplished on a simple loop of track with a siding or two. Setting out cars at these locations to be loaded or unloaded is considered operations. set up two trains, a passenger and a freight, and you have another dimension of operations, sheduled passenger trains. 

Advanced stuff also comes into play when the layout becomes more complex. Will the layout operate with a single person (you) or will you invite friends over to run the layout? How will the trains be dispatched? What kind of train line up will be run? 
All of this can lead to many choices. Find the set that best works for you and modify it if nessicary. 

The modeler looking into operations has several books available through kalmbach publishing (perhaps through your local library) to guide them through every step of operations, far more space than I have here. 

If your looking into operations to test the waters, see if you can find a local group that has operating sessions. Some clubs such as the La Mesa Club in San Diego operate in prototypical form, and there are MANY home layouts setup to operate where a group of guys will come in once or twice a month and operate on the pike. Many funs hours to be had.   

The Shed Division MKII General operations
The Shed division MKII layout is your generic shelf layout with plenty of switching opportunity. When I designed the layout, I took notes from layouts built in the UK and Germany, where large basement or room sized layouts are quite rare, and these shelf layouts are much more common. The minimum radius of the layout is 14 3/4", limiting the equipment I can operate but allowing me to squeeze the layout in a corner pretty well. 

General operations of the Shed Division MKII involve staging two trains in the tracks just outside of the still un-named town and running locals into town, switching as necessary. I don't use any form of car routing yet, still just a choice made by the crew as to what to pick up and drop off. 
Click to enlarge
A general overview of the MKII layout. The layout poses an interesting puzzle for crews as the staging tracks are longer than the passing siding used for run around moves. Also, if the crew is servicing any industry in the lower section of the diagram, they have a limit of an engine and three cars on the Switching lead to clear the switch. This all makes an interesting challenge for visitors running the layout, as moves must be planned in advance, or you can get yourself stuck and have to put some cars back in the sidings to get out.