There's kind of three levels, from my understand.
If you've got an Android phone, then full support for Google Voice is built in.
If you've got an iPhone (like I do), then you've got a few different options. There's a Google Voice app, and that can handle everything—sending and receiving text messages, sending and receiving calls, voice mail, and so on. Anything you do from inside that app will go through your Google Voice number, so they'll never see your real number.
Then, you can do a few other things. For example, I also have text messages from Google Voice go to an email address I have specifically for my phone. If I reply to that email, then the person on the other end will receive it as a text message from my GV number. I think there are also some other ways you can handle making outgoing calls, but I always just use the Google Voice app.
Finally, if your mobile phone isn't a smartphone, you can at least have Google Voice forward calls and text messages to your phone. That way, you aren't giving out your real phone number. However, to then make calls out and send text from the GV number, your only option might be through the website. If you go on the GV site, you can do all of that kind of stuff.
That site is also where you do all of the setting up as to how you want things forwarded, where to forward them to, what level of screening you have on your calls (I have it set so that if an incoming number isn't in my address book, they have to say their name and I get to screen the call first), and so on.