I did a thick sandwich course at University, but I had friends who did a thin sandwich. A thick sandwich means that I spent two years at university. I spent one year working in London, and then I came back to university for another year of study in my fourth year. My friends that did a thin sandwich course spent the first year at university, six months of their second year at university, six months on work placement. In the third year, they did the same, six months at university, six months on work placement and then back to university for a whole year in the fourth year. I did one long placement, other people at the same university and doing the same course did two six month placements. The silly term sandwich course just means that your degree has an element of work experience or working somewhere else, working abroad, studying abroad or volunteering abroad already built-in to it.
The advantage to doing a sandwich course is that you get work experience. As soon as you come out of university, you have that bit of experience that you can talk about in your CV. Your placement employer might offer you a job after university, and you might get paid for your work experience. You may get to travel and live somewhere completely different. You may get to be involved in entirely new, some fascinating research, which you wouldn't have done if you hadn't done placement. These types of courses are frequently offered in science, engineering, business, languages.
One of the downsides is that if you're doing a four-year course and all of your friends just doing a three year course, by the time you get back to university in the fourth year, you may find you don't know a lot of people. Now, this wasn't a problem for me because 90% of the people that I knew, people that I was friends with, where doing four-year courses. So when we all came back to university for the fourth year, it was just like our first year. So if you know you want to do a work placement, working abroad for a bit, look for a course that has that beforehand. You can apply to do work experience after you've started your course, but they're not always guaranteed.