Oregano (Origanum vulgare, sometimes listed with marjoram as Origanum majorana) is also called wild marjoram. It is a perennial common in southern Europe in dry copses and on hedge-banks, with many stout stems 30–80 centimetres (12–31 in) high, bearing short-stalked, somewhat ovate leaves and clusters of purple flowers. It has a stronger flavor than marjoram.
Pot marjoram or Cretan oregano (Origanum onites) has similar uses to marjoram.
Hardy marjoram or French marjoram, a cross of marjoram with oregano, is much more resistant to cold, but is slightly less sweet. Origanum pulchellum is known as showy marjoram or showy oregano. (source: wikipedia)
The key flavour in oregano comes from the chemical carvacrol however there are many other chemicals in oregano which give it its distinctive taste. The proportions differ depending not only on the variety being grown but also in the soil and weather conditions. Some people compare the flavour of oregano (and marjoram) to that of thyme and there is more than a little truth in this. The chemical thymol is present to varying degrees in oregano and this is also the key chemical in thyme. If you want to sample a strong tasting oregano then the variety Origanum vulgare subsp. hirtum, commonly called Greek oregano, has high levels of carvacol.
Marjoram (sweet marjoram is best for cooking purposes) is a much more mellow tasting herb with a sweeter tasting pine flavour and to a lesser degree citrus and clove flavours. Many cooks prefer marjoram over oregano because it provides a background flavours rather than dominant ones. If you analyse the contents of dried oregano which you buy in the supermarkets it will contain not only oregano but also some marjoram. Taste is a personal matter but often a mixture of both herbs is the ideal flavouring for many recipes.
Sabinene hydrate is the key chemical which flavours marjoram although it shares many of the chemicals present in oregano to varying degrees. A teaspoon of dried marjoram contains 2 calories and 0.04 of fat. (source: RHS)