Consists of many things. For a start, more chemistry than GCSE biology. Though don't worry, it's not pure chemistry, chemistry related to biology, eg different bonds between water, carbohydrates, etc. What happens when molecules eg two proteins join. That sort of thing.
It's also a lot more hands on, you'll do more lab work, some with microscopes, some without, eg I got to dissect a sheep's heart. Very fun! Depending on your course you may also have a field trip, we had one for AS and it was about ecology and we had to do our A2 coursework based around it.
There are definitely more concepts to be learned, you'll cover far more things in depth, especially things you haven't done at GCSE, eg muscles, DNA, immune system. Then there are things you've done at GCSE but you'll do again in a far more complex way...eg respiration, photosynthesis.
I'd highly recommend it, it's an excellent and well respected subject and will show you can handle both sciences and non science subjects well