Bad things happen, my friends. Lots of bad things.
Our bodies depend on Earth's atmosphere to survive. Not just for oxygen. The atmosphere protects us from ultraviolet radiation. It regulates temperatures. And it wraps us in a cocoon of lovely, lovely air pressure. In the vacuum of outer space, all that protection goes away. The most serious danger is asphyxiation. After about fifteen seconds, your body has used up all the oxygen in your blood, and your brain loses consciousness.
Now, you might be thinking - I can hold my breath for a minute! That might work underwater or under our atmosphere, but in outer space, there's no outside pressure. With no outside pressure air expands and can rupture the tissues in your lungs. Meanwhile, the water in your body turns into vapor beneath your skin, causing the mother of all bloating. Youll swell to about twice your normal size. Your body wont explode like you see in some movies, but you will be in a world of hurt.
After a few minutes, if the lack of oxygen hasn't killed you, the damage from depressurization will.
Outer space has other hazards - freezing temperatures, lethal radiation. But here's the good news you won't live long enough to have to worry about them. Despite all the dangers, if youre rescued within 60 seconds, you'd probably survive. You wouldnt be in good shape, but youd be alive. And you'll have first hand knowledge of why you never go to outer space, without dressing up in a nice looking space suit.