SEO- Search Engine optimization
SEM- Search Engine Marketing.
Both are related to Website promotion terms which can be used using various tasks and which is possible to give good Rank as well good traffic for a website.

IN SEO Main tasks
- Submission and Linkbuilding
- 1way,2way,3way

IN SEM-
- Keyword development
- Website and Landing page development

Both are really good in terms of website promotions.