The metal detector is one of the primary workhorses of the treasure hunter, particularly of the hobbyist THer. There are three primary types of detectors. Frequency shift designs include beat frequency oscillator (BFO), phase-locked loop (PLL), and off-resonance. Induction balance (IB) encompasses transmit-receive (TR) and very low frequency (VLF) detectors, as well as two-box detectors that are sometimes referred to as RF (radio-frequency). Finally, there are pulse induction (PI) designs.
Check out the Projects page if you're interested in building a detector, or if you just want to know what a typical circuit looks like. Most DIYers are building PI detectors because they are relatively easy to construct (including the coil), and a homebrew can give pretty decent performance.
The Patents page is especially valuable if you want to know the gory details about detector design, especially the commercial ones. The patents are packed with valuable information, but it's tough reading.