I'm not sure about your 'different types' comment, but Ginseng is a stimulant that helps to improve attention span, relieve prolonged fatigue or tiredness and generally make you more alert. Ginseng is particularly good when combined with another product called Ginko Biloba, which is the bark of a tree ground down. Ginko appears to help stimulate the production of new brain cells and will aid with memory and retention. Like most complimentery therapies a period of roughly six weeks on a decent dosage is required before you feel the effects. Also with Ginseng, because it is a stimulant it might not be wise to take it over long periods of time. Consult a GP to make sure you are OK to use these sorts of things first.