There are many forms of citrus, but most are hybrids. Actually there are only about 5 original species, and the rest are hybrids. For example, the common Sweet Orange is a Pomello and Mandarin mix. Grapefruits are Oranges (Pomello x Mandarin) mixed back to a Pomello. Lemons use a Sour Orange (still an orange, but a different line), mixed with a Citron. Limes are Lemons mixed with Key Limes, which are in turn Micrantha fruit mixed with a Citron.

Pomello: This is one of the main building blocks of most citrus. It usually lends toward creating larger fruits.

Mandarin: The other main base fruit for most sweeter citrus. This helps create more of the flavor for oranges, clementines, and the tangors. Many Mandarins for sale have trace amounts of Pomello in them, but were back crossed to the original Mandarin for flavor while gaining some size.

Kumquat: These small very tart fruits have little pulp, but help create some of the sour hybrids.

Sweet Orange: Oranges come in many lines and variations, but all have roughly equal parts Pomello and Mandarin roots.

Valencia & Heirloom: There are many selective production lines that have been cultivated to improve size or taste. These few are still traditional Navel Oranges.

Cara Cara Orange: Still a sweet orange, but has a pinker and more flavorful pulp.

Bitter/Sour Orange: While still equal parts Pomello and Mandarin, this is an alternate line that has more seeds, and isn't as sweet. Useful for producing other hybrids.

Blood Orange: A mutation of a Sweet Orange, but has since been reproduced from clippings. Pulp is deep crimson, and has a stronger taste.

Key Lime: A cross between two other base fruits, the Citron and Micrantha (Papeda) result in a very sour and small fruit. They grow on thorny bushes and are good for pie.

Grapefruit: A triple hybrid created when an Orange is crossed back to a Pomello for the larger size and tartness.

Clementine: Like the Grapefruit, the Clementine is crossed back to the other parent, the Mandarin. This results in a smaller snack-sized fruit with more of the original Orange's flavor, but with the Mandarin's easier to peel rind.

Lemons: A cross between a Bitter Orange and a Citron. Both source fruits lend to the extreme sourness.

Meyer Lemon: Similar to a traditional Lemon, but uses a Sweet Orange instead of a Bitter. This creates a fruit that doesn't need as much cane sugar, and often has a more orange colored rind.

Tangelo: A Grapefruit and Mandarin Cross, which is essentially 62.5% Mandarin, and 37.5% Pommello. As such, it's similar to a Sweet Orange, but has a distinctive taste.

Sumo Citrus: A cross between a Mandarin and Clementine, it's overwhelmingly Mandarin lending to it's almost identical taste with an easy to peel rind, but a larger size due to the Pomello influence.

UGLI Fruit / Jamacain Hope: Technically a Tangelo, but has more of a rind of a Mandarin, but inner pulp of a very bland Pomello.

Persian Lime: A keylime crossed with a Lemon. This creates a lemon shaped fruit, but smaller and more sour.

Ruby Tango: One of the more complex hybrids, it has mainly still Pomello and Mandarin heritage, but uses the Blood Orange mutation to give it some color. The Clementine parent provides some extra flavor.