It means a component or strand of a module, such as Language or Literature in module GM1001. Groups are the units we allocate you to when we subdivide the students taking GM1001 for tutorial or seminar teaching. For Language and Literature your groups are likely to consist of between 8 and 10 students. Some groups will meet in the class hour, when all students are able to attend. Other groups have to meet at times outside the class hour.
Lectures are formal presentations to all students taking a module.
The lecturer will give a prepared talk, and you should take notes so that you have a record of the important points. Your notes will be useful when you come to write an essay or revise for an exam. You may be given a handout with illustrative or additional material or even a summary of the lecture. Seminar is a teaching group of roughly 12-20 students. It will tend to be quite formally organised, with students preparing tasks or presentations individually or in pairs.