BB Messenger will work with anyone with a BB and BB messenger. It is native to the BB regardless of their email service provider.
Sms Text is a text message that is sent to phone via the phone number.
MMS is multimedia, like a picture message or video message sent to a cell phone number.
Google talk will work with anyone who is one google talk regardless of whether they are on a BB or not. BB to computer, computer to computer, BB to BB. As long as you are signed in using GoogleTalk you will see your friends who are signed into GoogleTalk. You will have no idea if they are at their computer or on the BB.
There is a sticky thread that deals with removing the duplicate sent message when using Gmail. This is when the message you just sent ends up in your inbox as if you sent it to yourself. If that's what you are talking about, then search for it.
If you are talking about delivery and read confirmations, there is a setting that someone already mentioned to remove it.
Email is an electronic message that is sent to someone else's email address. You can use your BB to send and receive emails to the account that you have it setup with. You would send someone an email in much the same way you would use www.Gmail.com
to send an email.
Welcome to the forums. You should check out the search feature. You will learn a lot. BlackBerry - BlackBerry 101