Lecture is basically what you're used to in high school, minus the time you get to work on homework and talk with friends. It's basically a prof standing in front of a typically large group of students blabbering on for 50 minutes or so about some topic.
Seminar is typically either a review done by a TA of the material recently covered in lecture or a discussion led by a TA of material covered in lecture and/or required readings. It is done in a small-group setting (30 or so students at most).
If OP is refering to UTSG courses, then the meaning of seminar is very different from matt's explanation.
Here in UofT, seminar is a smaller class (<25 students)taught by a prof. They usually have no exams, but there are slightly more weekly assignments. It's definitely easier to get good marks in a seminar than taking a lecture, so I highly recommend taking a seminar in first year.
haha it is different at every school, cause for us, seminars are just special, interesting lectures that have no credit worth. you just sit there and listen to speakers. or if youre in a specific program in engineering, youre given coop information, a talk about international exchange opportunities, etc.