GeoGebra is free, multiplatform dynamic mathematics software that joins geometry, algebra and calculus.
What does multiplatform mean?
Multiplatform means it works on both PCs and MACs.
What does dynamic mean?
Dynamic means that the user can interact with the computer.
Where does the name GeoGebra come from?
Take the first three letters of GEOmetry and put it together with the last five letters of AlGEBRA:
GEO  metry 
Al  GEBRA 
How is it pronounced?
Dr. Fisher pronounces it like Algebra, except with a ‘Geo’ at the beginning. (Both G's are soft.)
Here are names that will be used for various GeoGebra elements in subsequent tutorials and worksheets.
(Some of these names are eversoslightly different than the names that GeoGebra uses.)
GEOGEBRA WINDOW:  
TOOLBAR:  
MOVE tool:  
POINT tool:  
LINE tool:  
SEGMENT tool:  (in the pulldown menu beneath the LINE tool) 
PERPENDICULAR LINE tool:  
PARALLEL LINE tool:  (in the pulldown menu beneath the PERPENDICULAR LINE tool) 
POLYGON tool:  
CIRCLE tool:  
ANGLE tool:  
REFLECT ABOUT LINE tool:  
SLIDER tool:  
MOVE DRAWING PAD tool:  
MENU: 
Notation like ‘OptionsAngle unitRadians’ is used to mean the following:

ALGEBRA WINDOW:  
GEOMETRY WINDOW (or DRAWING PAD):  
INPUT FIELD:  
DEGREE MENU: 
(This menu contains the degree symbol, constants, functions, and relations. I call it the ‘degree menu’ because that symbol was originally what showed.) 
GREEK LETTER MENU:  
COMMAND MENU: 