What were German U boats?

The only thing that was "special" is that they received a lot of publicity; as they were an enemy (or the enemy). Otherwise, they were like most other submarines, whether they were the US Gato class, British, Japanese, Italian, or Russian.

German submarines fought a war of Guerre de Course, that is Germany pursued a war against merchant shipping (Commerce Raiding). Nations that do not have a traditionally strong battle fleet (battleships & cruisers) will, by necessity, have to fight a war of Guerre de Course.