The web pages of the US National Geospatial-Intelligence Agency (NGA) provide a Geodesy for the Layman guide. In addition, the web pages of the Bureau Gravimetrique International (BGI) contain training materials on geodesy at a high standard.
For a standard text book on geodesy, see:
For UK people, the Ordnance Survey can provide free of charge 'A Guide to Coordinate Systems in Great Britain' by Dr. P. Davies which is somewhat UK-biased but provides an excellent introduction to the geoid, heights 'above sea level', map coordinates etc. Jump to the Ordnance Survey web page and in the search box type 'coordinates' and follow the prompts. (If in difficulty contact the PSMSL for a copy).
Other information, mostly for geodesy professionals, may be obtained via the web page of the International Association of Geodesy.
GPS is used in conjunction with tide gauges in order to separate vertical land movements from the 'real' sea level variations contained within the records of 'relative' sea level change recorded by the gauges. Land movements can result from tectonic motion, glacial isostatic adjustment or serious subsidence. If GPS (or another space geodetic technique or absolute gravity) is used to measure the vertical land movements at points near to the tide gauges, then this signal can be removed from the gauge measurements, improving estimates of 'real' or 'absolute' sea level changes. GPS at tide gauges is also used to refer the tide gauge data to a global, external reference system (such as the International Terrestrial Reference Frame) for comparison to data from satellite radar altimetry.
The Workshop Report of:
Neilan, R., Van Scoy, P.A. and Woodworth, P.L. (eds). 1998. Proceedings of the workshop on methods for monitoring sea level: GPS and tide gauge benchmark monitoring and GPS altimeter calibration. Workshop organised by the IGS and PSMSL, Jet Propulsion Laboratory, 17-18 March 1997. 202pp.
provides important background information from the JPL GPS Workshop in 1997 on the use of GPS at tide gauge sites for measuring changes in the heights of benchmarks for studies of long term trends and for altimeter calibration. The report is available in both paper and electronic forms. For paper versions, email firstname.lastname@example.org. An electronic version is contained in two Adobe Acrobat files on this disk:
availability of permanant GPS receivers near to tide gauges has recently (July 2007) been updated on behalf of GLOSS, PSMSL, EUREF and other bodies.
There is a lot of basic training information on GPS available on the web. For a good introduction, see the notes of the University of Colorado (formerly presented by the University of Texas).
There are also explanations of GPS in manufacturers' web pages and in journals available on the web, for example see:
This file (Word 97 format) consists of a short set of notes on the basics of levelling. The file is based on lecture notes by Professor Charles Merry of the University of Cape Town at the 1998 GLOSS Training Course at UCT.