One basic way of categorizing estimates is as top down or bottom up.
Top down estimates use heuristics or rules of thumb to get the basic estimates.
Bottom up estimates use the WBS to identify the work packages and estimate each for time, cost, and resources. It gets a more precise estimate since the work is broken into smaller pieces.
It is often good to do both. If the project team does a bottom up estimate, the project manager should have a top down estimate ready for comparison purposes - it is useful to determine if the team estimate is in the right ballpark and identify if there might be gaps or areas that need more planning.