A deferral is usually granted if you have received tuition in all your courses, but are unable to sit your examinations or submit your dissertation due to extenuating circumstances.
An Interruption of Studies is usually granted if, due to medical or extenuating circumstances, you have missed a significant amount of teaching and would like to interrupt your studies for one academic year, e.g. if you interrupt your studies in the Lent term, you will resume your studies in the following Lent term. You would therefore continue your programme in the subsequent year and pay the Lent and Summer term fees at that year's rate.
Further information is available from the Interruption of Studies website.