It can be hard to predict how long a fire will last and how hot it will burn if you are unfamiliar with the wood types used in that fire. Different species of woods will burn hotter or cooler than others, just as some will burn more quickly or slowly. Knowing the type of wood you are using and having a reasonable expectation of its performance will help you use the correct wood for the task at hand to get what you need out of a fire-nothing more, nothing less.
In situations where you lack choices, any fire is better than no fire at all for cooking and keeping warm. Whenever possible, however, it is much more beneficial to know your woods and be able to make selections based on that knowledge. In the comforts of home, for example, you may prepare a ribeye on the stove with the goal of it being medium rare by cooking it on high heat for four minutes on each side. Over a fire composed of wood that does not generate much heat, that guideline no longer applies. Granted you will probably not be eating a ribeye in the wilderness, but if you are eating squirrel or duck, those meats still need to be cooked through. In the absence of a meat thermometer (which would be nice but probably will not exist), the next best thing is going to be knowing what wood type can be used to best generate an adequately heated fire, or knowing how long you must cook something over a slow burning flame that is not quite as hot.
Heat is generally measured in the form of BTU's or British Thermal Units. One BTU is the equivalent of a unit of heat required to one pound of water one degree Fahrenheit at one atmosphere of pressure. This translates one BTU into the equivalent to 251.997 calories. One BTU is comparable to 1055 joules. A one million BTU hot water boiler is capable of heating an outdoor pool. It takes 20 million BTU to float a hot air balloon for one hour.
Examples of woods that burn at low heat (12-20 million BTU's) are:
Examples of woods that burn at high heat (21-28 million BTU's) are:
Big Leaf Maple
While knowing your woods is helpful, a good rule of thumb is that denser woods will burn hotter. When you need a hot flame, go with the densest wood you can find. If a less powerful flame is preferable, opt for a less dense wood. Don't forget to use seasoned woods as seasoning will help you have a hotter, more efficient flame. Seasoned woods require less energy to burn because there is less water contained inside of them.
Whether you are trying to cook a quick dinner over an open flame or huddling by a fire for warmth, selecting a wood species that corresponds with the job at hand can save you a lot of time. Rather than constantly having to add logs to your fire to keep it going or having to search for water to douse a fire that is too much, you can take the right steps with the appropriate logs and get the job done right the first time.