The word rasayana is made up of two Sanskrit words, rasa (nutrition) and ayana (transportation in the body). Thus, rasayana denotes the combination of multi ingredients (herbal and minerals) as well as single herb to be used in an appropriate manner (Vardhmana Pippali). Kitchen herbs and mineral or the salts are part of the balanced diet. Rasayanas are claimed to improve vitality, rejuvenate body tissues, improve immunity against some (bacteria, virus) and prevent aging. Rasayanas may act in a variety of ways by improving the nutritional value of the food, digestion and absorption of nutrients, transportation of nutrients to tissues, bioavailability of nutrients, metabolism of nutrients in tissues, immunity, and by cleaning the srotas (microcirculatory channels or pores) which improve uptake of micronutrients.

Rasayanas are classified into three categories:

(1) Ajasrika rasayana (nutrition, dietary) is taken regularly with food as nutrition

(2) Naimittika rasayana is indicated to promote vitality in a particular disease.

(3) Kamya rasayana (normal health promoter) is indicated to

improve vigor, vitality, and to make a healthy person feel better

Kamya rasayana is further classified into three categories:

(a) Pranakamya rasayana (promoter of life vitality and longevity)

(b) Medhakamya rasayana (promoter of intellect)

(c) Srikamya rasayana (promoter of skin complexion and luster)

Ayurvedic physicians should be consulted to determine the appropriate rasayana required depending upon the health needs.
Rasayanas are the Ayurvedic equivalent to modern dietary supplements. 

It is understandable that vitamins, minerals, and biologically active steroids, alkaloids, glycosides, tannins, and a variety of antioxidants present in rasayanas would be a good source in providing necessary nutrients to the body. Sometimes an overdose of pure vitamins in modern dietary supplements can cause serious side effects.

There is virtually no possibility of an overdose of any specific vitamin or mineral from a rasayana.

Brief classification as per Classical texts of Ayurveda

KutipraveshikaStay in a very specialized manner in the kuti (hut or home)
KutipraveshikaStay in his/her own home while doing his or her normal duties
Kamya rasayana RasayanaUsed for benefit of specific system (e.g., for bone marrow Withania somnifera and Asparagus racemosus)
Naimittika rasayana RasayanaGiven only for partial period for certain number of days
Agasrik rasayana RasayanaCommonly used to keep the process of formation of vital tissue (e.g., milk)
Sharira rasayana RasayanaWorking on physical levels (e.g., Emblica officinalis)