A drifting buoy drifts around at the surface of the ocean. Most measure pressure, sea-surface temperature and, by their changing position, ocean currents:
http://www.aoml.noaa.gov/phod/dac/gdp_drifter.php
An Argo float drifts around at depth (usually 1000m), periodically diving down to 2000m and then rising up to the surface measuring temperature and salinity profiles as they go. They then sink back to 1000m and wait till the next cycle, which typically has a 10-day repeat.
http://www.argo.ucsd.edu/How_Argo_floats.html
Diagram Monkey 