For images, probe navigation, and other info about unmanned exploration, this is the place to go. JPL is run under contract for NASA by the nearby California Institute of Technology, unlike the NASA centers above. This distinction is subtle but critical. JPL has different requirements for unsolicited research proposals and summer hires. For instance in the latter, an SF 171 is useless.
Employees are Caltech employees, contractors, and for the most part have similar responsibilities.
They offer an alternative to funding after other NASA Centers. These are facilities of the US Government and are regarded with some degree of economic sensitivity. Centers cannot directly return information without high Center approval. Allow at least 1 month for clearance.