Pannier racks are a great way of creating extra storage on your bike which can be used to carry everything from your weekly shopping right through to camping supplies on longer touring rides. Pannier racks come in a number of different types and selecting the right rack for your needs will depend on 3 main factors:

The amount you want to carry - different pannier racks will have different weight ratings so it’s important to select a model that is appropriate for the items you wish to carry. The last thing you’ll want is for your rack to fail in the middle of a long touring ride.
Your bike’s frame - In order to mount a traditional style pannier rack, your bike’s frame will need to have special mounting eyelets for the rack to bolt onto. If your frame doesn’t have these then you’ll need to get a pannier rack that has a different attachment method, such as a seatpost or seatstays clamp.
Your type of bike - Some traditional style pannier racks won’t be able to be mounted onto modern road or mountain bikes, particularly those with front and/ or rear suspension. You’ll also want to look for a pannier rack that is suitable for use with disc brakes should your bike have them.

It is also a good idea to check whether the model you’re looking at is compatible with your pannier bags if you already own them. Finally, keep in mind that you should always evenly distribute your load’s weight evenly on either side of your bike in order to avoid undesirable handling issues.