What does a 2.1 honours degree mean?

How can you find out, will it be on the uni's website?? just to be more specific, I'm talking about any science degrees....

It should be somewhere on each university's website. The final classification is usually a weighted average of your final two years (eg. 50-50, 40-60, 10-40-50), though it all depends on the university in question though (and even then, it can vary from department to department). Pretty much all count 60%-69.99% as a 2:1 mark though.

For example, if your university used a 50:50 split. And your results were something like this:

Second Year
Module A: 63%
Module B: 72%
Module C: 66.5%
Module D: 70%
Your average second year mark would be 67.88% (2.1)

Third Year:
Module A: 67%
Module B: 76%
Module C: 72%
Module D: 71%
Your average third year mark would be 71.5% (1st).

Your final average mark over your final two years would thus be 69.69% and you'd receive a 2.1 Honours Degree. Some universities hold a 'viva voce' (a kind of general oral exam) for candidates who score near the top of the classification boundaries, and can have their classification upgraded if they can demonstrate an outstanding knowledge of their modules.