Dream Land can also refer to the area in which the following stages are located: Dream Land (from Super Smash Bros.); Fountain of Dreams, Green Greens, Past Stages: Dream Land (all from Super Smash Bros. Melee); and Battleship Halberd (from Super Smash Bros. Brawl).
The stage was first introduced in Super Smash Bros.. This is also the stage where a group of Kirbys is fought. This is a rather simple stage, being similar to other stages like Super Smash Bros. Melee's Battlefield and Fountain of Dreams where it has 3 platforms floating over a main one. Whispy Woods, a recurring boss in the Kirby series, is also in the middle of the stage, facing left. He will blow toward one side of the stage every so often to mess up the battle. His wind seems a little less powerful than it is in Melee's similar stage, Green Greens, where he can blow characters off the ledge. This stage is legal in Smash 64 competitive play. It is worthy to note as well that Dreamland is the only legal stage in the Japanese ruleset. Bronto Burts and King Dedede can also be seen flying in the far background.
Dream Land is Kirby's home stage, and it is also the stage where Ness is fought while being unlocked (this could be because both characters come from games developed by Smash 64 developer HAL Laboratory).
Past Stages: Dream Land
Dream Land is also available in Super Smash Bros. Melee under this name, and is one of the three unlockable Past Stages from Super Smash Bros. This stage is unlocked by beating Target Test with every character. Like all Past Stages, it is completly unchanged.
In competitive play
This stage is universally neutral and is one of three starters (along with Hyrule Castle and Congo Jungle.) It is the most common stage played along with Hyrule Castle in the American ruleset, though it is the only legal stage in the Japanese ruleset. The general given reason why it is legal is that the majority of the cast benefit heavily on this stage (outside of Link and Fox to a lesser extent), due to its small size and potential for characters to start early combos to rack up high damage or get early gimps, making it less campy than Hyrule or Congo. Competitively, the stage's only "hazard", the wind, also affects the stage much less than the tornados on Hyrule or the barrels on Congo. It is the 2nd most common doubles stage as well, though the larger size of Hyrule is preferred for this. It is also a semi-common stage in the Peruvian ruleset, but not as common as Hyrule.
The stage isn't much different than a "standard stage", with a basic layout and only one type of hazard (the wind), which barely affects gameplay. The stage is very good for quick 0-death combos, as well as characters who can combo into an edgeguard/gimp easily, such as Pikachu or Captain Falcon. Link and Fox are considered worse on this stage than Hyrule due to the much smaller size, giving less space to projectile camp, which they use to disrupt opponent approaches and setup the eventual close combo when the opponent is vulnerable.
This stage is neutral in Melee as well. The stage is slightly larger in Melee than it is in Smash 64, along with a much higher ceiling. This is generally considered the best stage for Peach, Jigglypuff, and Ganondorf, as well as Samus against certain characters.
The very high ceiling of the stage is a pretty big advantage for floaty characters, making them significantly less vulnerable to quick vertical KO combos, such as Fox's up throw→up aerial. Floaty characters benefit from this against characters such as Marth, who relies on his up tilt for KOs against floaties, outside of edgeguards (which are usually difficult to perform against floaties as well). It is additionally good for characters with strong vertical endurance, mostly Captain Falcon, Donkey Kong, and Falco.
According to a now outdated Japanese poll on Melee (2004), it was considered the most neutral stage ever. (This poll obviously did not include any stages from Super Smash Bros. Brawl.)
Dream Land is the generic term used for Kirby's homeland, paralleling Mario's Mushroom Kingdom or Link's Hyrule. In Kirby's Dream Land the first boss is Whispy Woods, who is an apple tree. Whispy Woods has also been the first boss in Kirby Super Star's Spring Breeze. In this stage Whispy Woods resembles the Kirby Super Star version of him. One of the attacks that Whispy Woods use is he sometime blows out puffs of air that would cause damage. In this stage Whispy Woods blows out air, but this does not cause damage. King Dedede is the main recurring villain in the Kirby series. One of the attacks King Dedede uses in Kirby's Adventure and Kirby Super Star is he puffs up and tries to ram into Kirby. King Dedede can be seen puffed up flying in the background of this stage. The overall design of King Dedede looks similar to his appearance in Kirby's Dream Land 3. A common enemy in throughout the Kirby series are Bronto Burts. A Bronto Burt can also be seen flying across the background of this stage. 
Description from the Instruction Booklet
Beware the wind that blows from the Whisper Woods - it just might blow you away.