I 'm quoting few Hadiths to clearify the meaning of the Hadith you have mentioned.
1. Sunan Abudawud (35:4265) Narrated AbuMusa:
The Prophet (peace_be_upon_him) said: This people of mine is one to which mercy is shown. It will have no punishment in the next world, but its punishment in this world will be trials, earthquakes and being killed.
2. Sahih Muslim (020:4706)
It has been narrated on the authority of Abu Huraira (through another chain of transmitters) that the Messenger of Allah (may peace be upon him) said: Whom do you consider to be a martyr among you? They (the Companions) said: Messenger, of Allah, one who is slain in the way of Allah is a martyr. He said: Then (if this is the definition of a martyr) the martyrs of my Umma will be small in number. They asked: Messenger of Allah, who are they? He said: One who is slain in the way of Allah is a martyr; one who dies in the way of Allah, is a martyr; one who dies of plague is a martyr; one who dies of cholera is a martyr. Ibn Miqsam said: I testify the truth of your father's statement (with regard to this tradition) that the Holy Prophet (may peace be upon him) said: One who is drowned is a martyr.
3. Sahih Bukhari (11:624) Narrated Abu Huraira:
Allah's Apostle said, "While a man was going on a way, he saw a thorny branch and removed it from the way and Allah became pleased by his action and forgave him for that." Then the Prophet said, "Five are martyrs: One who dies of plague, one who dies of an abdominal disease, one who dies of drowning, one who is buried alive (and) dies and one who is killed in Allah's cause." (The Prophet further said, "If the people knew the reward for pronouncing the Adhan and for standing in the first row (in the congregational prayer) and found no other way to get it except by drawing lots they would do so, and if they knew the reward of offering the Zuhr prayer early (in its stated time), they would race for it and they knew the reward for 'Isha' and Fajr prayers in congregation, they would attend them even if they were to crawl')
4. Sunan Abudawud (14:2535) Narrated Hasana' daughter of Mu'awiyah:
She reported on the authority of her paternal uncle: I asked the Prophet (peace_be_upon_him): Who are in Paradise? He replied: Prophets are in Paradise, martyrs are in Paradise, infants are in Paradise and children buried alive are in Paradise.
5. Malik's Muwatta(8: 8.2.6)
Yahya related to me from Malik from Sumayy, the mawla of Abu Bakr ibn Abd ar-Rahman from Abu Salih from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, "If a man who is walking along a road finds a branch of thorns on the road and removes it, Allah thanks him for doing it and forgives him." He also said, "martyrs are five: the one killed by a plague, the one killed by a disease of the belly, the one who drowns, the one killed by a collapsing building, and the martyr in the path of Allah.' He also said, "If people knew what there was in the call to prayer and the first row, and they could find no other way except to draw lots for it, they would draw lots for it. And if they knew what there was in doing dhuhr at its time, they would race each other to it. And if they knew what there was in the prayers of isha and maghrib, they would come to them even if they had to crawl .