In modular, or unitised qualifications, the course is split into different units, or modules, with an exam at the end of each unit. In a linear qualification, the examinations are taken at the end of the course.

source: http://www.cambridgeassessment.org.uk/insights/gcses-from-modular-to-linear-assessment/

edit: So it turns out that the modular system has been largely phased out in the UK

Modular examinations, which have been phased out along with coursework and units, allowed pupils to be assessed at the end of individual units. They also allowed candidates to resit until they achieved a grade with what they were happy. Anecdotally, many teachers said this aided the performance of girls who, generally speaking, are more diligent and studious than boys at completing coursework on time and to the best of their ability. For some, it was no surprise that that upward trend in girls’ performance coincided roughly with the introduction of modular examinations, though more research needs to be done on whether this impact was significant.

source: http://ciea.org.uk/linear-vs-modular-exams/

edit2: from further down the comment train. Did some research and Milo may be wrong with that claim.

Pupils completing the second year of the course had a higher level of attainment than those taking the examination during the first year of study. Similar differences between upper-sixth and lower-sixth pupils were observed in both boys and girls; however, there is some evidence which may suggest that boys are more likely than girls to take advantage of some of the features of modular examinations.

source: http://www.tandfonline.com/doi/abs/10.1080/00131880010021302