"texting 101" help needed

[marklyn]

I'm fairly new to texting and have discovered the benefits of texting to friends and family. I'm a little confused about 'how' to text to my friends who are on different carriers (other than sprint) for example.
It doesn't appear that I can always text only to a phone number in all cases, or am I wrong?
For example, for me receive a text from an email account, it has to be to "my_number@sprintmessaging.com" instead of just my phone number but am I correct in assuming that if someone is texting me from their mobile phone they would then just use my phone number?

I'd appreciate a primer on texting formats to carriers from an email account or from my cell phone, when to use just the phone number and when more is needed.