There are 3 maths courses, in order of difficulty:
Further Maths - SL only - take as a group 6 subject along with Mathematics HL
Mathematics HL - Do all the stuff in Maths SL, some additional HL stuff and then a Option topic from a selection
Mathematics SL - previously called Maths Methods SL. Like Maths HL but easier with no additional Option
Maths Studies - SL only - AFAIK it focusses on application of maths while Maths HL and SL has more pure maths stuff
Further Maths basically do all the Maths HL Options and then some more.