Fellowships
A fellowship is a program that you can undertake during, or at the end of your specialty training. Fellowships can last from anywhere between 6 months and 2 years (sometimes even longer). The point of a fellowship is to gain expertise in a specific field within a specialty. For example, if you are training in plastics, you might do a fellowship in hand surgery. Or, if you are training in neurology, you might do a fellowship in movement disorders. Many people go the US for fellowships because a) a fellowship in a different country can offer experiences you may not get here and b) a lot of pioneering treatments and techniques come from US hospitals and research institutions.
Because fellowships take place later in training, there is less emphasis on scoring as highly as possible in Step 1 (which is heavily pre-clinical). Lots of registrars who apply for fellowships and take Step 1 later in their careers may just need a passing grade, so that they can get ECFMG certification. Of course, the Step 2 CK and CS scores will matter a bit more, because they are clinically oriented. By this point in your career, other things also count for more, such as previous experience, research output, teaching, audit, professional responsibilities, letters of recommendation, why you want the fellowship etc.
As a medical student myself, I don’t have much information on fellowship applications, so it’s best to speak to an ex-US fellow if you are planning this far ahead. It’s possible to take Step 1 as a student so that it’s out of the way when you start working as a doctor. But there is a time limit, as this quote from the ECFMG website explains: “If you do not pass all required Steps and Step Components within a maximum of seven years, your earliest USMLE passing performance will no longer be valid for ECFMG Certification”. So, sitting Step 1 won’t count for anything if you don’t do the other exams within 7 years. But once you have your ECFMG certificate, it’s valid forever.