you can earn a set amount throughout the year before you get taxed on it
depending on how you get paid they will either divide it into 52 weeks for weekly pay and tax you accordingly or they will divide it by 12 and again, tax you accordingly

so if you were to earn £12k a year as a basic and your tax code was 360 and you got paid monthly, you would get a grand a month and out of that you would pay anything you earnt over £300 at the normal rate of tax (22% i think) and everything you earn as national insurance (10% i think)

they are the basics but i also have bupa cover and other stuff like pensions and the like going from my pay which adds anothe £100 a month or so to the total deductions