Now I'll go over thermostats and thermometers.
Thermostats are devices that use a temperature probe to measure the amount of heat that a heat source is outputting. They them vary the amount of power going to said heat source to keep the temperature in the desired range that you set it to. There are 2 major types of thermostats. On/off style thermostats, and Proportional thermostats.
On/Off Style Thermostats:
On/Off style thermostats such as true Hydrofarm, Ranco, and Johnsons thermostats are cheap but lack accuracy. they work the same way a home A/C thermostat works. They run the heat source at 100% power until the set temperature is achieved. The thermostat then turns the heat source completely off until the "swing" is reached. The swing refers to how many degrees below the set temperature the thermostat lets the temperature fall before turning the heat source back on again. This is why they are inaccurate.
Pros:
Cheap
Will do the job
Cons:
Inaccurate
Usually don't have some of the additional safety features that proportional thermostats have.
Here are some good on/off style thermostats:
Hydrofarm:
http://www.amazon.com/Hydrofarm-MTPR.../dp/B000NZZG3S
Ranco:
http://www.reptilebasics.com/ranco-etc-111000-pre-wired
Proportional thermostats:
Proportional thermostats such as the Herpstat, Helix, and VE-200/300 are extremely accurate and much safer than on/off style thermostats do to more advanced design and high build quality. This does come at a price though. The cheapest I have seen a proportional thermostat sell for (new) is $99. The cost is worth it though. These thermostats especially the herpstat have far more capabilities than on/off thermostats and as I already said are more accurate and safer. Instead on simply running the heat source at 100% power or 0% power proportional thermostats adjust the amount of power in small increments. this allows them to maintain the correct temperature very accurately (usually to a fraction of a degree off of the set temperature)
Pros:
Safe
Accurate
More features (such as the ability to automatically drop temperatures at programed times/regulate humidity/turn lights on/off/etc.)
Cons:
Cost
Here are somme good proportional thermostats:
Herpstat:
http://spyderrobotics.com/
VE line:
http://www.reptilebasics.com/thermostats (note the VE-100 is not a proportional thermostat)
Helix:
http://www.helixcontrols.com/DBS1000.htm