Leather gloves are recommended. Leather may cost a little extra, but will last longer and provide better support. For fitness classes this might not an issue, but for boxing training it is more important.
VELCRO OR LACES
The classic style boxing gloves that you always see in prize fights are tied on with laces. Just like lacing up your shoes, laces provide a nice snug fit.
The advantage to buying a pair of boxing gloves with Velcro straps, is that you can easily put on the gloves and remove them yourself. If you have ever tried to lace a pair of gloves when nobody is around, it is almost impossible. For your average person and general training purposes, gloves with Velcro straps are very acceptable.
SIZES AND WEIGHT
Boxing gloves come in small, medium and large sizes. The average man will need a large size. The average women is medium. This can vary between manufactures, but is fairly standard.
Boxing gloves weights range from 10-20 ounces. In a professional boxing match, the fighters usually wear 10oz gloves.
For training purposes, most gloves weigh 12oz, 14oz, and 16 oz. The heavier the glove, the more protection it offers. For beginners, 14oz and 16oz gloves are recommended. For sparring purposes, 16oz boxing gloves are standard.
See: Choosing Your Boxing Glove Size
THUMB ATTACHMENT
(THUMB-LOCK)
Most boxing gloves manufactured these days have a thumb attachment feature. This was a very good idea as it prevents a lot of thumb injuries and accidental eye gouges.
BUYER BEWARE
When buying a pair of boxing gloves, don't just buy the cheapest pair that most sports stores carry in stock. Maybe they only cost $29 and look great, but are not much more than a novelty item. The most famous companies make their bread and butter selling worthless equipment.
DON'T FORGET
Always wear handwraps