PDK = Process Design Kit.
It is circuit design environment. Normally PDK is created by foundry, which included all design-releted information of one process such as: simulation model, Pcell(parameters cell : defined by design rule), DRC/LVS/RCX technology files. Schematic-driven-layout and auto-rounting is regarded as the most useful function of PDK which can save many time on layout.
Skill is language used in many Cadence's tools. PDK is created by skill.
Attachment is introduction of Chartered's PDK from Cadence's website. FYI.