Hard drive rotation speed eg 7200rpm = number of revolutions per minute
Colour mode originally stated number of colours available eg 256 colours, more commonly number of bits used to store the colour information for each pixel eg 16 bit colour can display 65,356 colours (2 to the power of 16)
Scan frequency in relation to monitors is the number of times per second the electron guns write the entire screen and is expressed in Hz (hertz) eg 70Hz is 70 times per second.
RJ = registered jack and generally describes communications connectors eg RJ11 is the little one used on telephones, RJ45 the bigger one used in networking
DB = D shell connectors (no idea what the B stands for - anyone?)ie the pins are surrounded by a trapezoid metal case (one side shorter than the other) to prevent improper connection. Used for serial, parallel, video and other connections.
You'll learn more if you look these things up - howstuffworks.com is a good place to start. Any good pc maintenance book should help.