Fasting
When you fast, you do not eat any food for a period of time. The time period can be from 12 hours to a week. Extreme fasting can go even longer. Fasting is mentioned in the bible so it is not a new idea.
Several doctors recommend a fast of of 12 hours or more one day a week. These allows your body to rest. You probably should drink a liquid like water or tea without sugar a couple times during the fast.
If you are planning on using fasting to lose weight, it may be a better Idea to do juicing instead. Also, if you have Renards Syndrome, you may want to try juicing instead.
Detoxing
Detoxing usually uses herbs or various other types of cleanses to remove toxins from the body and allow the body to heal itself. If you want to detox, find a good proven plan.
There many reasons to detox. You feel bad, you cannot sleep, you want to lose weight, or you want to cut down on eating too much sugar.
Here is a link to some detox plans.
Here is some information on the Master Cleanse,
Juicing
Juicing Is a form of detoxing, but you drink juices and liquids instead of eating or fasting. These liquids usually are fruits or vegetables that have been ground up in a blender or juicer until they are a liquid. In addition to getting more fruits and vegetables into your diet, juicing has several other benefits.
With juicing, if you do it too long it can cause some gastric upset.