Lessons are usually something that happens repeatedly, and are at a lower level of education than seminars. For example, you can take a weekly guitar lesson or English lessons of dancing lessons. Seminars are usually only a single meeting. An expert in a certain topic will come to town and do a one day seminar. For example, a famous guitarist may give seminar, or a scientist or businessman may do a seminar.

If you attend a lesson, then you will probably be sitting in a classroom with chairs and tables, and you will be surrounded by other students (maybe 10 - 30 other students). You might have an English lesson, a Maths Lesson, a History lesson etc. The lesson can cover a broad range of topics within the one topic. So, if you have a Chinese class, then you might learn some new words or new grammatical structures. You might answer questions that the teacher asks. You might split into smaller groups and do work together. On the other hand, a seminar is more professional. A seminar might be in an auditorium at a university, and there might be 100 people there. A seminar is almost always about a specific topic. So, there might be a seminar on how tense and aspect are portrayed in Chinese, or it might be about how a doctor can use a specific technique to treat a specific disease. Seminars can go for an hour, or they can go for several hours. Generally, there is specific time to ask questions, say 10 minutes or 30 minutes depending on the length of the seminar. It's less interactive than a lesson. The presenter speaks for the whole time, and there is rarely any user engagement with the audience. Some synonyms: Lesson = class Seminar = lecture (university), workshop (community), conference, symposium Hope that makes it a bot clearer :)