If you're a COD Elite member, when you're in a lobby and you see a Plus Sign (+) on a little LEGO-looking person to the right of someone's name, this indicates that the person shares a common interest (Group) with you. To see what your common group is, click their name and choose "View Common Groups" (or whatever it actually says .. something like that).
No, call signs were not introduced until Modern Warfare 2.
Yes, there is online multiplayer on Call of Duty 4: Modern Warfare.
By completing challenges.
Call of Duty World at War, Modern Warfare, Modern Warfare 2, Black Ops, and Modern Warfare 3 all have online multiplayer.
Callsigns give you a little Phrase and picture above your Name for online play. http://callofduty.wikia.com/wiki/Callsigns
yes
Modern Warfare 2 is a better game if you are going to play online.
No there is no cheats but there is glitches
No
yes
well call of duty and modern warfare are the same it is called Call of Duty: Modern warfare and then came call of duty: modern warfare 2
yup