If you getting into the cycling groove, and have a few things to carry, then you need to get a good understanding of what bags are out there. There is a lot of variety, and a number of positions of where the bag can be stowed.
Typically, before choosing a cycling bag, you should have an idea as to how much you will be carrying and what the purpose of your ride is commonly for. Naturally the serious hard core endurance road bike cyclist wouldn’t hoist a pannier bag at the back of their bike, whilst a work commuter wouldn’t squeeze his shirts and pants in a saddle bag.
Saddle bag

Saddle bags are small bags that reside underneath the seat of a bicycle. The seat is often referred to as the saddle, hence the name saddle bag. For many road bike cyclists, a saddle bag is used to store the accessories required for bike maintenance. These may include, tubes, levers, repair kits, and other tools. Other items such as wallet, keys and phone could also be stored in a saddle bag, however most riders, tend to keep these in the back pockets of their jerseys for easy reach and frequent use.
Saddle bags come in various shapes and sizes. Most are strap on, and loop through the metal rods underneath the saddle. Some saddle bags allow for lights to be attached at the back, providing another option to place a back light for the cyclist.
Handlebar bag

Handlebar bags are shaped differently to saddle bags and are less common for road cyclists due to the drag that they create in the air. Handlebar bags suit the recreational rider, who wants that the simple access of storing items in front of them. They generally hold a little bit more than the saddle bag, but don’t expect it to be able to hold work clothes, and other larger items.
Pannier

A pannier hangs off the back of a bicycle. A metal holder is usually used to support the pannier and is often used for work commuters. They are a great way to store larger items, and can be hooked at the back of a road bike. Office workers will often place their shirts, pants and shoes in them. They offer a lot of stability which lends to a more comfortable ride, when compared to wearing a backpack.
Trunk bag

A trunk bag is more suited for mountain bike riders. They are a lot chunkier than a saddle bag, and sits more outward from the saddle of the bike. It can therefore hold a lot more than the smaller saddle bag, hence not particularly used by road bike cyclists.