Dedicated resources are those resources working solely on projects. A dedicated resource is 100% allocated on the same project until his role on that project is finished (after that he's moved to another project). A dedicated resource cannot work on two projects at the same time. Quite often the whole project team consists of dedicated resources, and all these resources start at the beginning of the project and remain allocated to the same project until it's finished.
Another type of resources is matrix resources, where the resource works on several projects at the same time. For example, on a given day, he works for 4 hours on project A, 2 hours on project B, and 2 hours on project C.
Of course, dedicated resources are much more efficient than matrix resources because they will not experience the overhead of switching tasks.
On the other hand, matrix resources rarely get bored, because they are not working on the same project all the time (unlike dedicated resources).