The STEP is a series of three exams, it is usually seen as a basis for top universities to select their candidates. It is a very challenging, respected admissions test which can potentially land you a place for Maths at Cambridge. STEP has 3 papers : I,II and III. I is the easier one and II and III have relatively the same difficulty (harder than I).
It is aimed at the top 2% of the students taking A-Level Maths, so in order to take it you should be an A* student, but that is in no way enough to excel at STEP. Even the most intelligent within those 2% percentile will have some sort of struggle when it comes to preparation for the STEP.
The purpose of STEP is to replicate how the Mathematics is at university, getting used to solving problems which at first seem difficult but with contant preservance and hardwork they can be cracked. The STEP questions are styled how you would expect undegradad mathematics courses examinations to be. It's a strong benchmark for motivation !
STEP I and II covers the A-Level Maths syllabus (C3 and C4), while STEP III covers further maths (FP2 and FP3). The majority of questions are based around Pure Maths hence you should focus on the pure modules.
The structure of the questions are : 8 pure, 3 mechanics and 2 probability and statistics. Candidates choose 6 questions to answer. The timing for the exam is 3 hours across all papers.
More than anything it is important you "understand key principles", not just how to answer questions. The step questions are twisted in unusual ways, and often require you to apply the maths.
I've been tutoring some of my A-level maths students to crack the STEP !! If you have any queries, get in touch : )