Undergraduate students at Cambridge attend lectures but are also taught in small group tutorials, called supervisions. Supervisions are one hour sessions where typically two students meet with one supervisor, usually a postdoc, lecturer or graduate students, for an hour to discuss lectures from a particular module or course. Supervisors are generally expected to organise assignments and give feedback on any completed work.
Colleges pay supervisors an hourly wage for every contact hour of supervision but not for preparation time. The hourly rates for supervisions are determined centrally by the University and vary depending on the number of students in a group. Supervisors are generally paid at the end of the term after they have submitted a report on their students using a system called CamCors. If you are interested in supervising, the University now requires that you attend a courses outlining the expectations for new supervisors. Courses are offered frequently through professional development: http://www.admin.cam.ac.uk/offices/hr/cppd/
Generally, supervisions are organised through each college by the college's Director of Studies, or more informally DoS. If you are interested in supervising, send an email, including information about yourself and an academic CV to the following email list: email@example.com - This email list reaches all the DoS's arranging supervisions in the colleges around Cambridge. If you are going to email the list, it helps to include a list of the potential undergraduate courses you would be interested in supervising. For this, make sure you look at the options in 1A and 1B Natural Sciences http://www.cam.ac.uk/about/natscitripos/links.html