All Bachelors' degrees are 'taught degrees', obtained through studying existing facts and methodologies.
All Doctorates (other than in medicine) are 'research degrees', where the candidate has to research within a topic and add something new to existing knowledge about it.
Masters' degrees fall into the middle and can be either taught degrees or research degrees (or sometimes a combination of both).
So some Masters' degrees are obtained through students simply studying areas of a subject which weren't covered in the courses for their Bachelor's degrees, whereas other students embark upon original research in order to get MA (or MSC, etc) after their names.
At Oxford and Cambridge no studies of any kind are required to get a Master's degree. Graduates holding a Bachelor's degree can simply pay a fee to 'upgrade' to a Master's degree. So, while MA(Oxon) might look impressive it's actually only equivalent to BA elsewhere.