Remote sensing refers to a set of technologies that allows us to "sense" or study the earth, other planets, and space objects indirectly. It includes aerial photography and photogrammetry, satellite imagery, global positioning systems (GPS), radar, sonar, and LIDAR (Light Detection And Ranging) technologies.
This extensive tutorial works through all the aspects of remote sensing including the history, image processing and interpretation, and various applications (among which are geologic, vegetation, urban and land use).
From NASA Landsat Science.
GIS.com - this site provides answers to questions such as: What is GIS? Why use GIS? as well as explanations on how to use and analyse geographic information systems. It also provides GIS users with resources to help them in their work.
CSIRO COSSA/EOC - the Office of Space Science & Applications and the Earth Observation Centre are units of CSIRO that coordinate and promote activities in national and international space science activities and earth observation with the use of remotely sensed data. Current priority areas of application include sustainable natural resource management, managing production systems, including agriculture, forestry and fisheries and management of emergencies such as fires and floods.
CSIRO Environment Group - leading edge environmental research that is helping Australia build a more sustainable future.
Geographic Information Systems - an introduction to GIS from the U.S. Geological Survey that teaches you how GIS works and has been applied through history to the present day.
Geoscience Australia - is the national agency for geoscience research and geospatial information.