Currently Being ModeratedMar 25, 2013 1:48 PM (in response to Asilah)
If you have just an iPhone 4 then you an only iMessage.
It will then depend which OS she has.
If she has Mountain Lion (OS X 10.8.x) then she will have the Mac version of Messages which will have iMessages capabilities.
If she has an earlier OS then she could AIM or Jabber IM you if you had a suitable App on your iPhone or logged in with Safari.
You would both need either AIM valid Names or Jabber based ones.
Although Apple IDs issued from Apple (those ending in @mac.com, @me.com and @icloud.com are valid AIM screen Names the last two issued via iCloud cannot be used outside of iChat 6 or Messages due to the way the app logs in to Apple as well as AIM to confirm that AIM can "read" the password.
This stops the use of other AIM client apps or web Browser logins.
Google email IDs ending in @gmail.com or @googlemail.com are valid Jabber ID s once you enable "Talk" on your Google settings.
Facebook IDs and enabling "Chat" are also Jabber IDs
In Snow Leopard (OS X 10.6.x) you can download the FaceTime App which would allow you to Video between the iPhone and the Mac.
That also applies to Lion and Mountain Lion that have the FaceTime App included.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously