Tree

Trees are the most important structure in Minecraft, being the renewable source of wood.

Oak
Oak trees are the first kind ever introduced into Minecraft. Their wood is the most well-recognized. While their leaves originally dropped apples, this function has been migrated over to a fruit tree variety. They have the most shape variety out of all tree types, coming in small, large, "balloon", and swamp. Swamp oaks are covered in vines.

Spruce
Spruce trees have dark wood, and are often found in colder climates. They also have high variance in shape. A two-by-two square of saplings can grow into a mega spruce tree, which are excellent sources of wood.

Birch
Birch trees have the lightest wood color. Their shape is always the same as the small Oak shape. Their variance is limited, and they are fairly rare.

Jungle
Exclusive to the Jungle biome, Jungle trees are the first to have a "mega" size, formed by a 2x2 square of saplings. Their bark has a distinct pattern, and their planks are slightly orange in comparison to Oak. Mega Jungle trees generate with vines.

Acacia
Exclusive to the Savanna, Acacias grow distinctly (it's best to check the Minecraft wiki for more info on this). Their bark is a greyish color, and their wood is very orange.

Dark Oak
Exclusive to the Roofed Forest, Dark Oaks are restricted to a "Mega" size. They have the darkest bark and the darkest planks. Like normal Oaks, they used to drop apples, and were actually a better source, as they consistently had lots of leaves.

Fruit Trees
A special variety, fruit trees have many subvarieties. Their bark is just slightly brighter than Oak's, and same goes for their planks.

Apple
Apple trees act somewhat like Oak trees did, by being the source of apples. Instead of a 1/200 chance per leaf block, apple trees have two main leaves varieties. Ordinary ones, with no visible apples, have a 1/90 chance per block, while ones with visible apples can be right-clicked to harvest 1-3 apples. Ordinary leaf blocks have a 1/10 chance to spawn as an appled block instead, if they have at least one face touching air. Otherwise, they have a timer that lasts 400 ticks (20 seconds); every 400 ticks, a block has a 1/30 chance to "grow" apples. They are fairly rare in Forest biomes, but any decently-sized forest will have a good bunch of them. They have their own biome full of them, Apple Forest.

Lemon
Lemon trees function very much like apple trees, only with a 1/32 chance to "grow" lemons. Lemons are a new fruit item. They also have their own biome, Lemon Forest. They're a tad rarer than apple trees.

Lime
Limes are very, very similar to lemons. Their trees function the same. Their own biome, Lime Forest, includes small amounts of lemon trees, and vice versa. Lemons are also a new fruit item.

Orange
A third citrus tree, orange trees are found almost purely in Jungles, with minimal populations in forests. Their own biome, the Orange Forest, is also quite rare. Oranges are, once again, a new fruit item.