Color settings???

In the settings option for monitor resolution I have four color options.

1) 16 colors
2) 256 colors
3) high color 16 bit
4) true color 32 bit

My monitor came from the factory set at high color 16 bit.
What are the differences between the settings?

truiz9902@hotmail.com

Dimension 4100
P3 933MHz
256 MB 133MHz SDRAM
40GB Ultra ATA 7200 rpm. HD
19" P991 .24AG monitor(extra lines free)
32MB NVIDIA TNT2 M64 AGP GC
SB LIVE! Value digital w/ Music Match Plus
8x/4x/32x LG CED-8080B CD-R/RW
Creative 12x DVD
HK-695 surround sound w/sub
OS Windows ME
Serv tag #HTXS701