Sanskrit word for Mars is Kuja. Angaraka is another name for it. Dosha means defect--Kuja Dosha means defect because of Mars. Along with Saadhe-saati, Rahu-kalam and Kala-Sarpa Yoga, it's, in my opinion, the most talked about combination in horoscopes in Indian context. Most people are afraid of it and think that it must be something very horrible because it's an obstacle in way of their harmonious married lives and some worships must be performed to appease deities for getting rid of evil effects.
Fear always arises out of ignorance and superstition. There are just twelve houses in a horoscope to delineate all the matters related to life-narratives of natives, therefore it's pretty obvious that a planet would be found in all twelve houses if you look through enough number of charts. The Kuja-Dosha is not very rare because Mars situated in 1, 2, 4, 7, 8 and 12th houses causes this combination--you would find it in at least 50 percent charts. Further, in order to delineate this combination you should examine it from all three--that's from Moon, Lagna and Venus. In some cases Surya-Lagna is stronger than the previous three--that should also be taken into the account. Thus you would find that majority of charts would have this combination--therefore it's to the wise to take reckoning from all three charts--if all three or two of them are tainted--consider the native as heavily Manglika.
The Psychology of Kuja-Dosha is very simple to understand. Mars signifies aggression, leader-ship, competence, physical strength, vitality and yogic practices in horoscopes. When it over-energizes the marriage house or houses which are adverse to the harmony of marriage--it causes difficulties for one's marriage partner. Mars in a horoscope, devoid of any soft aspects and in retrogression or debilitation might hint that the native has Karmic burden related to Grahastha-Ashrama to cope with. It's not easy to really change the Karmic script without being aware of one's behavior--without being meditative.
It must also be noted that Mars' 4th and 8th aspects are 100% whereas 7th is only 50%--therefore a Mars located in the 7th house is most malefic, then the ones with Mars in the 4th or 8th houses, as they have their 4th and 8th aspects to the 7th house, respectively.
Learned ones also say that Jupiter's full aspect on Mars cancels this dosha. Rahu located in a kendra(1,4,7,10) from Mars is also supposed to cancel this dosha--whereas Saturn's aspect to an already Manglik 7th house makes it doubly Manglik.