If you don't do honours, you can't specialize (ie go into biochem, or bio, or chem, or whatever). You can get a BSc in Life Sciences, Physical Sciences, Math/Stats or Environmental Sciences, but not in chemistry or biochem or physics or anything like that. They're much more general degrees.
If you do honours BSc, you have so many more options for majors, and your degree becomes a lot more specific. Your major is your program of study, so for example, my major is Chemical Biology, since that's the program I'm in. I started off in Life Sci, and then chose to major in ChemBio after first year. People do a major for many reasons--the job they want after graduation might be in a certain field, they might have a very strong interest in a specific field, some really interesting courses are only available to those in a certain program, etc. If you are in an honours BSc program, and then get into med school for your 4th year (after having completed 3 years), you can apply to switch from honours to general, so that you still get a degree before going to med school. If you're already in med school, it won't really matter what type of degree you have, you'll have gotten the benefit of honours, and if med school fails, I think you can go back and do your final year to upgrade your degree to an honours BSc if you need to. You can also get an honours BSc in Life Sci if you want to stay somewhat general (if you have a wide range of interests or want a lot of elective space), but still want an honours degree.
You can minor for the same reasons you would do a major, except it's implied that you have less expertise in your minor than your major, since you take less courses in that area. You'd generally use your electives towards a minor, so you shouldn't need to take any 'extra' courses. You need a certain number of credits in a specific area to minor in it, and it usually includes specific mandatory courses. You can go to the department's section of the course calendar to find out any minor requirements. The course calendar for this year is here:
http://registrar.mcmaster.ca /CALEN...ent/index.html To find out the minor requirements for biochem (as an example), go to that link, click on 'Faculties, Programs and Schools', then 'Faculty of Science', then 'Department of Biochemistry and Biomedical Sciences', then 'Minor in Biochemistry'.