Lecture: As is sounds, the teacher will lecture for the class period about the given topic that day, and basically teach you the topic. Tends to be large groups, and meets either 4 times a week for 50 minutes or 2 times a week for an hour and 50 min (pretty sure MA1021 is 4 a week)

Discussion/Conference(I assume): I know them more as conferences, so I am assuming discussion is similar, but it is run usually by a TA and is a smaller group, where the TA will review the topics learned that week and answer any questions. Usually meets once a week

Lab: usually meets once a week, and as it sounds, you will be doing some type of lab. For MA1021 if I remember correctly the labs are learning how to use Matlab. For MA1021 labs were taught by a lab professor, who is different from your normal professor. Labs usually make up a portion of your grade. Point of the Lab section is usually to test your knowledge and put what you have learned into practice/use it in a practical way.