It's usually something like this:
1st class - 70% and above
2:1 - 60 - 69%
2:2 50-59%
Pass - 40%
Pass (no honours) - 35% -39% (I think this is what my uni does)
You will do a collection of modules that add up to a certain amount of credits that are required to achieve a degree, at the discretion of the Board of Examiners for your uni.
For a non-clinical 3 year degree, second year usually 'weighs less'. For example, second year counts towards 30% and third year 70% of the whole degree. First year often does not 'count' to the overall classification but a pass is required to move into the next year, but this varies at each institution.
Your grade will be taken as an average across each module. Using some basic maths and some scaling factors that usually remain unknown to students, that is how your grade is calculated.