World 1 (Super Mario World 2: Yoshi's Island)
From the Super Mario Wiki
World 1 is the first world of the game Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. The world contains eight normal levels and two secret levels. The world's theme is a grassy plain in spring.
 Levels of World 1
The following is a list of each level in World 1.
The first fort is an easy level. At the beginning Yoshi must be careful because some walls can stomp him. The level features the Burts, Podoboo and much more. The boss of the fort is a giant Burt that jumps really high. Yoshi must throw eggs on it to beat it.
This level is not too easy, it's a side-scrolling level so Yoshi must follow the path on your direction to go to the end. The level features the Donut Lifts as says the name.
This is an easy level with many Shy Guys on stilts. This special kind of Shy Guy can't be swallow by Yoshi. At the end, an 1-Up balloon will appear.
This is the first castle of the game. It's an easy level, however Yoshi must be careful with the spiked platforms. The boss is Salvo the Slime, a giant green Lemon Drop. Yoshi have to throw eggs on it to defeat it.
Available only in Super Mario Advance 3: Yoshi's Island. This is the first secret level in the game. As the name implies, Yoshi will have to traverse through cloud platforms and other types of airborne terrain most of the time.