Thermometers are turning up everywhere in today's kitchens in all shapes and sizes — digitals, instant-reads, probes for the oven and microwave. They're high-tech and easy to use.
Some thermometers are meant to stay in the food while it's cooking; others are not. Some are ideal for checking thin foods, like the digital. Others, like the large-dial thermometer many people use, are really meant for large roasts and whole chickens and turkeys. Using a food thermometer is the only sure way of knowing if your food has reached a high enough temperature to destroy foodborne bacteria. It is essential to use a food thermometer when cooking meat, poultry, and egg products to prevent undercooking, verify that food has reached a safe minimum internal temperature, and consequently, prevent foodborne illness.
Digital Food Thermometers

Thermocouple:
Of all food thermometers, thermocouple thermometers reach and display the final temperature the fastest – within 2 to 5 seconds. The temperature is indicated on a digital display. 
A thermocouple measures temperature at the junction of two fine wires located in the tip of the probe. Thermocouples used in scientific laboratories have very thin probes, similar to hypodermic needles, while others may have a thickness of 1/16 of an inch.
Since thermocouple thermometers respond so rapidly, the temperature can be quickly checked in a number of locations to ensure that the food is safely cooked. This is especially useful for cooking large foods, such as roasts or turkeys, when checking the temperature in more than one place is advised. The thin probe of the thermocouple also enables it to accurately read the temperature of thin foods such as hamburger patties, pork chops, and chicken breasts.
Thermocouples are not designed to remain in the food while it's cooking. They should be used near the end of the estimated cooking time to check for final cooking temperatures. To prevent overcooking, check the temperature before the food is expected to finish cooking.

Thermistors:
 Thermistor-style food thermometers use a resistor (a ceramic semiconductor bonded in the tip with temperature-sensitive epoxy) to measure temperature. The thickness of the probe is approximately 1/8 of an inch and takes roughly 10 seconds to register the temperature on the digital display. Since the semiconductor is in the tip, thermistors can measure temperature in thin foods, as well as thick foods. Because the center of a food is usually cooler than the outer surface, place the tip in the center of the thickest part of the food.
Thermistors are not designed to remain in the food while it's cooking. They should be used near the end of the estimated cooking time to check for final cooking temperatures. To prevent overcooking, check the temperature before the food is expected to finish cooking.

Oven Cord Thermometers:
This food thermometer allows the cook to check the temperature of food in the oven without opening the oven door. A base unit with a digital screen is attached to a thermistor-type food thermometer probe by a long metal cord. The probe is inserted into the food, and the cord extends from the oven to the base unit. The base can be placed on the counter or attached to the stovetop or oven door by a magnet. The thermometer is programmed for the desired temperature and beeps when it is reached. While designed for use in ovens, these thermometers can also be used to check foods cooking on the stove.  

Infrared Thermometer

Are for checking the surface temperature of the pot or pan that you are cooking in. Not to be used for checking the temperature of food you are cooking it will only tell you the temperature of food on the outside not on the inside. Thay are for checking  temperature of your griddle or skillet to see if they are ready or to hot to use.