An MSci is usually a 4 year undergraduate degree in a science subject; an MMath is essentially the same but specifically in mathematics. I believe each individual university can decide which it wishes to award, and there isn't necessarily a difference between the courses. Both are undergraduate degrees (undergraduate masters courses), but will carry more weight with employers than a BSc, for example, as you have studied for an extra year.
An MSc on the other hand is a one year postgraduate degree which you can do either after a BSc (or similar) or MSci/MMath degree. It won't be as hard if you have already done the undergraduate masters as opposed to just a BSc but won't cover exactly the same stuff and is generally considered to be higher qualification and therefore harder, mainly due to the the requirement for original research.
Hope that helps a bit!