Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah (PBUH) said, "The martyrs are of five kinds: One who dies of plague; one who dies of disease of his belly; the drowned; one who dies under the debris (of construction, etc.), and one who dies while fighting in the way of Allah."[Al-Bukhari and Muslim].
The Hadith mentioned four categories of people, besides those who were slain on the battlefield, whom Allah will, by His Special Grace, give on the Day of Judgement an award similar to martyrs on condition that they are true believers and practising Muslims. In some other Ahadith, certain other persons have also been mentioned who will be given the status of martyrs by Allah. There is no contradiction in these Ahadith for the reason that first of all the Prophet (PBUH) was told about five categories of martyrs which were disclosed by him. Subsequently Almighty Allah added some more people to them which were also mentioned by him. The real Shaheed is one who voluntarily gives his life for the sake of Allah provided that he wholeheartedly fights on the battlefield.