Implementation practice has it's own org structure, but the trajectory is very similar to a consultant track. For instance, in a consultant track you move from Associate, EM, AP to Partner. Implementation track is Coach, Leader, Senior Expert and Partner. The time to make the jump between each role is similar, but with the Implementation track there is less of the stress to make the promotion soon. There is more flexibility here. With recent changes in McKinsey's org structure, both these routes now reach the Partner track.
To explain the difference between the work they do, let me give an example. If there is a end-to-end transformation that involves a diagnostic, design and pilot implementation, usually this would be done by a consultant. If that pilot were to be successful and the client wanted to roll it in their other sites, an implementation consultant would come to the scene and help execute that. On average, implementation consultants stay longer in each project (6-9 months) compared to consultants (2-4 months). Having said that, there is a lot of flexibility in implementation consultants doing other kinds of projects. In one of my recent strategy studies, I staffed an implementation consultant and they were great.
Hope this helps!