New to GPS ? (the preferred term these days is “GNSS” include all the different satellite systems).

There are any number of web-based educational resources which can teach you about how it all works.  The presentation depth ranges from very solid descriptions using almost no math, to those with more esoteric details and lots of math.  Select one you are comfortable with.  Here are just a few that we feel are very noteworthy.

http://gnss.be/gnss_tutorial.php

http://www.navipedia.net/index.php/Main_Page

Several Educational Institutions also provide good overview briefings as well

http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

http://www.csr.utexas.edu/texas_pwv/midterm/gabor/gps.html

http://www-cdr.stanford.edu/dynamic/navigation/GPSIntro.pdf

http://www.wou.edu/~vanstem/321.S13/GPS.2.pdf

https://ocw.tudelft.nl/courses/satellite-navigation/

Need the details?  Get the official message and interface specifications.  Here are links to the more basic ones. Some of these documents are free, while some are sold.

GPS, (IS-GPS-200n is the current interface spec, also often called ICD-200)
http://www.gps.gov/technical/icwg/

GLONASS
ICD_GLONASS_5.1_(2008)_en  (In English)
http://russianspacesystems.ru/

RTCM SC104, various specifications for differential corrections and NTRIP
http://www.rtcm.org/differential-global-navigation-satellite–dgnss–standards.html

NMEA-183, a popular ASCII based protocol to express positional fixes
https://www.nmea.org/content/nmea_standards/nmea_0183_v_410.asp

And of course all vendors have unique protocol specs for their own devices which you can generally obtain from their web sites.

Nomenclature Hint:  The message structures in RTCM work are referred to as “Message Types” as in the fragment “…message type MT1004 has GPS observations in it…”  – by contrast the structures found in NMEA 0183 (or NEMA 2000) are referred to as “Sentences” as in  the fragment “…the rover device periodically sends a $GPGGA sentence back to the Caster to report position…“