Map type

The map Type is the general format the level generator used to create maps. Added in 0.31, it dramatically affected Indev mode in the availability of Water, Sand, and Gravel block types.

Water Warning
With the exception of Island, water that is generated on the map is the only water you can get (Unless the 2x2 infinite water glitch is done or an inventory hack is used). If the blocks next to the Water are removed (dug/mined out) the water will instantly begin to evaporate for the water on that level. The level below will remain unless a block next to it is altered. You may add blocks to water but you cannot remove any without triggering evaporation.

Comment to above: The 2nd sentence (addition by user), I have never heard or seen evaporation in Minecraft. Appears to be nonsense.

Island
The default map type. It's an Island. Water existing at the borders for as far out as you are willing to swim. Due to this, water is quite abundant and can be toyed with freely. The islands will have minor hills and cliffs.

Sand and Gravel are very common.

Water is quite abundant.

Floating
The second map type and possibly the most bizarre is "floating". In it, you will find multiple floating "islands". Each of these floats high enough that on "normal" view distance it appears as though there is no ground. Regardless of how far you jump down from, the fall will kill you unless you make a stair down to it. But there is no point as the entire "surface" is covered with Bedrock. It is important to note that gravity works strangely here, as you will sometimes find single-layers of sand/gravel floating without a support. Digging out or placing blocks next to these single-layers of sand/gravel will cause the entire layer to fall to the ground (sometimes taking you with it). This can pose an interesting challenge for building.

Sand and Gravel are fairly common.

Water is rare.

Flat
The third map type is kind of "boring" in comparison to the others. It's a flat land with no Water or Sand. Flowers and Trees can be found dotted around it, but with the exception of those and your Indev House it's barren.

Sand and Gravel are non-existent.

Water is non-existent.

Most common in online classic servers.

Inland
The fourth map type is a slightly hilly landscape. In a nut-shell, it's the Island map type with infinite flatland at its borders as opposed to water, causing the tiles that would normally be "under water" to be exposed.

Sand and Gravel are very common.

Water is rare.