I want to keep it short & simple, so it basically goes something like this...

1. A stranger posts on your Steam profile page asking for you to add them because they can't add you. (Already suspect.)

2. If you add them as a friend they won't contact you for a few days, then when they finally do they'll message you asking why you haven't accepted their offer with a link for you to click. (More Red flags.)

3. If you click the link you'll see a trade offer that asks nothing of your Steam inventory but they offer you something valuable instead, usually a CS:GO item. The thing is that in the sent link the user doesn't match up with the one who sent the link, whom you previously added as a friend. (Shady beyond reproach.)

4. If you accept the trade a pop-up window asking for your username and password comes up. Valve constantly warns you to avoid sharing your password with anyone. If you enter your credentials, your account and all of its items will be stolen.

Just avoid the first step altogether and save yourself some time and a migraine.

Stay safe out there, both in real life and on the Internet!