Pannier racks are the racks which attach to the rear and/or front of the bike which enable you to carry pannier bags. They are available in a multitude of shapes and sizes but all follow the same basic pattern allowing you to carry pannier bags without them coming into contact with the wheels. Use will determine which specific model you need with heavy duty touring racks being more robust than something for carrying light loads. Made of alloy, they are much lighter than they look but much stronger than you think.
They attach to the frame or fork with small bolts and you will need to have the correct eyelets on your bike for this. On the rear you will need mudguard eyes at the rear dropout and rack mounts on the seat tube while if using a front you will need mudguard eyelets and possibly rack mounts so are a very specific item.
Another rack increasing in popularity is one which attaches directly to the seatpost meaning you do not have to have any special frame or fork attachments. You may not be able to carry as much as traditional racks but for small loads perfect and the ease attachment makes it compatible for nearly all bikes