Penwith is the far western tip of Cornwall - an area bounded by Penzance in the south, St. Ives in the north and Lands End in the west. It has the highest concentration of ancient monuments and burial sites in mainland Britain.
It's a popular holiday destination, packed with dramatic coastal scenery, sandy beaches and fishing villages. As a result it's also packed with visitors during the high-season months of July and August.
The best times to visit Penwith are spring and autumn, when there are fewer visitors and the weather tends towards fresh, clear days. Then you can walk along the beachs and cliffs, breathing in the clean air and watching the Atlantic breakers crashing onto the rocks.
West Cornwall is where the Atlantic weather fronts make their UK landfall, so the weather is always changeable. It's adviseable to bring warm and waterproof clothing at any time of year, although the summer months can also be very hot, with temperatures 5 degrees or so above the UK average.
Of the towns in Penwith St. Ives, famous for its artists colony, is a lovely place to visit, but very crowded in summer. Penzance is more down to earth, but still filled with shops, eating places and galleries.