aplite: merely a fancy geology word for a fine-grained, biotite-poor granite with a sugary texture that commonly forms tabular intrusions (dikes and sills).

castle tor: (also castle koppie) a hill formed from a jumbled mass of fracture-bounded granite blocks.

dike: a kind of planar igneous intrusion that cuts across layers or bedding in rocks, or cuts unlayered rock. Many dikes are aplites or pegmatites.

exfoliation: a rock weathering process in which concentric slabs, plates, or shells of rock, from less than a centimeter to several meters in thickness are successively removed from the outer surface of a large rock outcrop. The process is caused by physical or chemical forces producing differential stresses within the rock, as by expansion of minerals as a result of near-surface chemical weathering, or by the release of confining pressure of a once deeply buried rocks as it is brought nearer to the surface by erosion. (I borrowed this definition from somewhere and now I've forgotten where, sorry, bad form)
Web site covering this topic.

exfoliation dome: a dome-shaped hill typically with a bare rock surface, typically developed in massive coarse-grained rocks (like granites) by the process of exfoliation. Other exfoliation domes include Stone Mountain, Georgia and Independence Rock, Wyoming. Half Dome in Yosemite is apparently half on an exfoliation dome (hence the name); one side appears to have been sheared away by a glacier.

magma: molten rock beneath the surface of the earth (molten rock which reaches the surface is called "lava").

pegmatite: yet another fancy geology word, this time referring to an igneous intrusion that has very large (several cm up to several meter) crystals. Not all pegmatites are granitic, but many are. Pegmatites tend to form late in the crystallization process of an igneous intrusion, and therefore are rich in strange elements that don't fit well into regular granite. These strange elements promote the formation of minerals such as tourmaline, beryl, topaz, allanite, apatite, zircon and many others.

Precambrian: A particular part of the geologic time scale. Literally, pre-Cambrian, as in all the time before the Cambrian period. Originally this term was applied to rocks too old to have fossils, until the paleontologists found rare instances of older fossils and the whole thing got more complicated. Now the Precambrian is considered to be the time before the development of animals with hard shells, or for the numerically inclined, the time before 540 million years ago. The Precambrian is broken up into the Archean (oldest) and Proterozoic (somewhat less old).

schlieren: a tabular zone in a granite with either more or less of some of the minerals in the surrounding granite, typically the dark (mafic) minerals

sill: a kind of planar igneous intrusion that has intruded parallel to layering or bedding in rocks.

U-Pb zircon geochronology: try a page on isotopic age dating

xenolith: a piece of the surrounding rock that was broken off and dropped into the granite (or any other igneous intrusion) while it was still molten, from xeno "foreign" and lith "rock".