You may be familiar with the Chinese concept of meridians, energy pathways in the body. EFT involves gently tapping on the end points of these meridians, while focusing on the problem, to settle energy disturbances in the meridians. As you tap, and the energy settles, the level of emotion will reduce.
Depending on the severity of the problem, you may get quick relief after a few minutes of "tapping", or you may need to "tap" on many different aspects of the problem. This is when it's helpful to work with a trained practitioner.
EFT is very easy to learn and apply yourself, for the simpler applications, and can be implemented straight away for immediate change. But, in the hands of a good practitioner, it is also incredibly powerful for deep and complex situations.
The theory behind EFT is that all emotional upsets are caused by disturbances in the body's energy system. Correct the disturbance, and the emotional upset dissipates. This doesn't mean that we don't feel any emotion, rather that we have a tool that can neutralize excessive emotion.
As an example, suppose you wanted to use EFT to eliminate a fear of spiders. Suppose that, currently, whenever you see a spider, you feel so frightened you need to run from the room. After working with EFT for a time, you might get to the point where you can scoop up a wayward spider in a jar and put it outside, without feeling any upset. However, you are likely to still be cautious about picking one up in your bare hand, if you didn't know whether it was poisonous or not. EFT gets rid of the excessive fear, but doesn't get rid of your sense of self preservation.
You might have a traumatic incident from your past, that still causes very distressing feelings when you think of it. After using EFT, you won't forget the incident totally, but may be able to think about it without getting upset.
You can use EFT if you have a fight with your boss or spouse, if your children have had a bad day school and need comfort, to get rid of addictions, to lose weight, to boost self esteem, to improve your business and your relationships, to reduce pain, to increase your energy, to reduce fears for the future and see your way more clearly forward, to name just a few uses.