What is the difference between the Project Life Cycle and the Software Development Life Cycle?
How do they fit together?
The project life cycle represents the processes use to manage the work while the SDLC actually represents the work of creating the software system. In the following figure the PLC is represented on the horizontal and the SDLC is represented on the vertical as the execution items.



In reality there is some overlap between the PLC and the SDLC but the above diagram will help you understand where the SDLC fits in within the PLC. During the planning phase you will select a SDLC. The SDLC phases usually represent milestones within your project plan. Often the SDLC phases will become levels in the Work Breakdown Structure. The SDLC represents the Software Development plan.
Selecting a properly documented SDLC makes project planning easy. Several SDLC have step by step guides identifying tasks, activities and deliverables to be completed. You might also be lucky enough to find a methodology identifying controls and execution strategies.

Here is an example of bi-dimensional project planning using SCRUM as a work model.