Typically the smallest and lightest bicycle pack, the seat bag is a great option for carrying essentials on quick rides. True to its name, this bag straps to the underside of the saddle, keeping it out of the way and safe from the wind. Look for different sizes in seat bags, from just enough room for a flat kit and multi-tool, to extra stash room for keys and a few energy gels.
A handlebar bag is the natural partner to a seat bag. While the bar bag is still fairly small, it provides instant-access to essential gear, such as maps, food and even arm warmers or a jacket. Combined with a seat pack, the handlebar bag can easily haul enough gear for a century or a day-long tour of the trails.
The saddlebags of bicycling, panniers convert your bike into a pack mule. These bags are fastened to a sturdy rack and hang on either side of the front or rear wheels. Many cyclists ride multi-day, unsupported tours with only the gear they can stuff into a pair of saddlebags. Unfortunately, the bike will feel about as limber as a ton of horseflesh with fully-laden panniers, so they are not practical for single-day rides.