What is an assassin snail?

I have a 10 gallon tank, which is now overun by snails. I purchased an Assasin snail and for more than a month of watching him, he has not done much! I dont think he has killed one snail ( I even put him in a bowl with five snails and they crawled all over him) is my assasin snail broken or what should I expect. How many Assasin's should inhabit a 10 gallon tank? and is there a way to sex them?