At last count, there are fifteen quingigillion different bike jerseys on the market. But choosing the right one, you'll be glad to know, is actually not as tricky a decision as it might seem.
Material
Yes, you could wear a cotton t-shirt, but after a couple of hours of it gradually getting heavier and soggier as it soaks up all your sweat, you'd probably prefer not to. And when you stop, you'll get cold quickly. Technical jerseys employ materials that wick sweat from your skin and transport it away so that, no matter how hard you ride, you'll stay dry.
The type of fabric used is a good indicator of the intended use. Short sleeved lightweight affairs are better suited to the high temperatures of the summer, or if you plan to race. Look for man-made fabrics that boast high wicking properties. In general, the thicker the fabric, the warmer the jersey - a slightly thicker, long-sleeved jersey is a very useful garment to have in your riding wardrobe.
Natural fibres, such as Merino wool, are popular too, and manage to combine comfort with naturally high levels of sweat removal. Whatever the fabric though, what you're looking for is one that wicks sweat effectively, removing moisture from the skin to keep you feeling dry.
Fit
Jerseys differ greatly in the fit department. For XC racing a tighter fit will be more desirable, whereas for more laid back riding, downhill/freeride and in the pub, a baggier fit may be preferable. Bear in mind that a jersey will stand a better chance to wicking sweat in the summer if it fits closer to the body.
Within those broad categories jerseys come in many shapes and sizes so it's worth trying some on to find one that fits. One manufacturer's idea of "close fitting" may not match up to another's, and sizing tends to be a law unto itself too.
There's many different approaches to making a jersey fit well, from tapered sections, multi-panels, seam placement and whether the fabric is stretchy, or not. A jersey with a lower back, or dropped tail, will prevent builders bum syndrome. Bear in mind, though, that more sophisticated construction usually means a higher price.
Features
Which features you look for in a jersey will depend on the type of riding you plan to do. Good length zips are great for hot weather riding and allow you to control your temperature. Mesh panels can also help in keeping cool.
Pockets around the back will be good for those riders who prefer not to carry everything they need for a ride in a hydration pack, and certainly will be high on the list of priorities for XC racers. Zipped pockets, while they may push the price up, can be handy places for safely storing valuables such as house keys. The traditional triple rear pockets are generally the preserve of close-fitting race jerseys - carrying things there in a looser jersey tends to turn things from baggy to saggy.
Elasticated waist bands can stop the jersey flapping about, and some will be decorated in reflective tabs for low light visibility. Detachable sleeves can lead to a versatile jersey too.
Women
Women are well catered for with specific designs, features and styles. Some will feature built-in sports bras constructed of lightweight but supportive fabrics such as highly breathable mesh, which are a perfect way of hiding unsightly bra straps. Sizing of women's garments tends to be even more random than men's, though - try before you buy.
Colours
We can ramble on endlessly about fabrics and fit and features, but what people tend to actually buy is colours and designs. There's no harm in looking good, of course, but don't be blinded (in some cases literally) by funky graphics and colours - it doesn't matter how "factory" you look if the jersey rubs under your arms or you boil alive wearing it...
