GCC -lm -lz -lrt options - what are they about?

I know that these are some common/basic libraries, but what do they mean exactly?

For example, I know, that -lm is some math library, but is this the standard math library or what?

-lz for compression? What compression?

And I don't have any idea what -lrt is.

What are these things?

• math library. Is it the same that we use when we include <cmath> or <math.h>?
• compress library. What does this mean? Does it provide some tools that we can use to compress files, or does it help the compiler/linker to do some compress things?

• -lz - is zlib, http://zlib.net/
• -lm - is the math library as you've worked out (implementation defined AFAIK)
• -lrt- provides POSIX realtime extensions: http://www.s-gms.ms.edus.si/cgi-bin/man-cgi?librt+3LIB