1. Resting (or Basal) Metabolic Rate (RMR)
2. Energy Activity Thermogenesis (EAT)
3. Non-Energy Activity Thermogenesis (NEAT)
4. Thermic Effect of Food (TEF)

RMR accounts for ~50-70% of your TDEE. This is basically if you just sat around all day and did nothing, what your body would burn. EAT is going to be what you burn through exercise, which should easy to comprehend. NEAT are things like taking the stairs instead of the elevator, parking further away from your office and walking, chewing gum, tapping your feet or hands, blinking, etc. TEF is the energy burned from digesting food.

The RMR/BMR calculators are not great. They may be directionally accurate but it is tough when our RMR is genetic and can vastly differ from person to person.

Here is the best technique to use to figure out where you are at (although long and drawn out).

1. Track your daily caloric intake as accurately as possible for 2 weeks (I prefer the MyFitnessPal app)
2. Weigh yourself every day taking your weekly average for comparison purposes.
2a. Weigh yourself at the same time/same routine daily. This is important to get accurate comparisons.
3. After two weeks you should get a good idea of what your body is doing with the calories you consume. Two weeks may be too short, but most people won't go longer just to try and figure this out.

I've been doing this for about 2 years and so I have a pretty good feel for where I need to be on calories consumed to either gain or lose weight.

Calculators are easy, but not always the most accurate.