If you look at the properties for the operator it will explain what each one does. A scan looks at all or a significant part of a table or index. A seek looks at a more limited range of the index. The scan generally walks the pages where as a seek generally uses the key structure to find information. That's it in a nutshell.
Index seek: Because your query is selective enough, you can go directly to the items you need through the index. Example: SELECT * FROM Employee WHERE EmpID = 12345 Index scan: Because your query isn't quite selective enough, so the whole index is being run through to find out which rows are required.