what do we need to know about a shaheed or a martyr?

Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him) was asked about a man who traveled by sea for the purpose of trade, and he was drowned – did he die as a shaheed? 

He replied, yes, he died as a shaheed, so long as he was not travelling for sinful purposes.

For it was narrated in a saheeh report that the Prophet (peace and blessings of Allaah be upon him) said:

“The one who drowns is a shaheed, the one who dies of a stomach disease is a shaheed, the one who is burnt to death is a shaheed, the one who dies of the plague is a shaheed, the woman who dies giving birth is a shaheed, and the one who is crushed by a collapsing building is a shaheed.”

And people killed in other ways were also mentioned.

Travelling by sea for the purpose of trade is permissible, if one thinks it is most likely to be safe. If it is not safe then he should not travel for the purpose of trade; if he did that [i.e., traveled when it was not safe to do so], then he has contributed to his own death, and in such a case we cannot say that he was a shaheed. And Allaah knows best.