To add to Kevin58's good advice, yeast fermentation is exothermic so what the air temperature around the carboy or fermentation vessel may be at is not the temperature that is inside the carboy (which is likely to be hotter during the most active and most important phase of fermentation to control). Since the fermenting wort presents a thermal mass which may heat up and cool down slowly, especially if you are counting on air exchange as being you mode of providing heat or chilling to the carboy, it is important to note the temperature of the carboy. The cheapest route to go is thermal strips, which will give you some indication, though these can often be crude in accuracy. You can go all the way up to a thermowell, which a thermistor or thermocouple is placed inside the well which sits down into the fermentation chamber. Much more accurate, but more expensive as well. Mid-range is a thermistor or thermocouple which can be applied to the outside and hooked up to either a display or a controller, such as Kevin referenced. These are best when the probe is in direct contact with the outside of the fermentation vessel and some form of insulation is covering the probe to shield it from the external air temp.