A seek means that you are looking for specific value(s) and the index provides you with the best way to do it. This also applies to specific ranges of data.As Lowell described it's very fast.

A scan indicates that the entire table/index is read in. A scan isn't always bad, because it it might just mean that the data is organized the way your query needs it (meaning - there's no data to exclude, and the data is organized the way the query optimizer needs it). It might however also mean that there IS a criteria, but that can't be satisfied using SEEKS. Because of the less specific nature of the operation, it tends to be a little slower, and thus often described as "not as good". It may however be the best operation specific to what you ask for.