All Saiyans are at this level unless they reach Super Saiyan. A saiyan is usually at this level when he or she is ether calm or powerless. This really is not a level.
There is a legend that every 1000 years a super saiyan will be born. Well it seems that that legend is wrong because a lot of the saiyans in our time, in the show, have reached that level. The transformation makes their hair go up and turns it golden yellow. It also increases strenght, speed, and muscle mass.
Super Saiyans: Goku, Vegeta, Trunks, Gohan, Goten, Gotenks, Gogeta, Vegeto, and Broli
The ultra super saiyan is in between a super saiyan and a super saiyan level two. When Trunks turned in to it to fight Cell he soon learned that with a huge muscle mass slowing him down a lot that he couldn't even touch Cell! All this level dose is increase strenth and muscle mass and slows you down some more hair stands up a bit.
Ultra Super Saiyans: Trunks, Goku, and Vegeta
A Super Saiyan Level Two is alot stronger then just a Super Saiyan and stronger then an Ultra Super Saiyan. The main difference between a Super Saiuan Ultra and a Level Two is speed. A Level Two is much faster then any level before it. A Super Saiyan Two's muscle mass increases and their hair becomes smaller strands and looks more spiked.
Super Saiyan Two's: Gohan, Vegeta, Goku, Gogeta, and Vegetto
Super Saiyan Level Three is stronger then any level before it. The power of this level is so strong it only will last for about 15 minutes. The only difference is in appearance. The difference between this level and level two is that the eyebrows disappear and a lot more hair grows, and it does not go up anymore: it goes down.
Super Saiyan Threes: Goku and Gotenks
Level four is by-far strongest level of Super Saiyan. And also my favorite. The only way to reach the level is to be in control after turning the Golden Ozaru. This IS the strongest level of them all.
Super Saiyan Level Fours: Goku, Vegeta, and Gogeta
This level is just a rumor (which is false) but let me tell you something about it. I know for a fact that right now in Japan there is no new series of DBZ called AF or DBM or anything else you might have heard! Even Funimation has said that. The truth about this picture is that it was a pic drawn by a fan to make people excited, or it was just fan art to go along with a fanfic. I have also heard that it was a comic in Italy made by a company not affiliated with Toei.
Super Saiyan Level Fives: None Yet!