Aim a gun: Just point and shoot, right? Line up the three dots, pull the trigger and call it a day. In the grand scheme of things, yes, that's pretty much it. But proper sight picture is one of those fundamental things that makes the difference between a shooter and someone that just plays with guns.
What do we mean when we say "sight picture"? I'll be honest, I haven't been able to find an official definition of it but it is essentially the complete view of the target when the sights are properly aligned and pointed to the correct location. This doesn't just mean line up the three dots and put it over what you want to shoot. It also involves having the right things in focus.
There are a lot of different configurations for gun sights. The most common, however, is the 3 dot sight. All of them, no matter what their setup, work on the same principle. Because the 3 dot sight is the most common, that's the one I'll be focusing on here. I honestly prefer the three dot over any of the others anyway.
If you're looking at your gun, the front sight is the one closest to the barrel opening. It's always going to be a single dot or line. At the back of the gun is the rear sights. The rear sites, at least in the 3 dot sights configuration, is going to have 2 dots with a gap between them.