Removable retainers are more hygienic, as they can be removed when you brush your teeth. However, you need a bit of self-discipline to wear them at night. Some people are forgetful, or are not so disciplined, so they may prefer fixed retainers. Fixed retainers may be bonded on two, six or eight teeth on their back surfaces, depending on the needs of each individual. Accurately adapting wires are bonded onto the teeth using bonding cement. Sometimes, the bonded retainers can break if one is not careful while eating hard foods. Some people tend to accumulate tartar around their teeth easily. For such individuals, I would recommend removable retainers, because tartar could build up around the bonded wires.
Removable retainers may be entirely plastic, or there may be wires incorporated. Those made of acrylic tend to last longer, although other kinds of plastic may do the job as well. If there are wires incorporated, the wires can be adjusted to move the teeth, if needed. The type of retainer you need depends on the arrangement of your teeth, your bite, cosmetic and other reasons.