Gender

A Gender determines wether the Pokémon is male or female in the Pokémon video games. Some Pokémon have a greater chance of being one gender than another, such as Blastoise which has a 87.5% chance of being male, but only a 12.5% chance of being female. There is also moves that will only work when Pokémon are certain genders. There were no genders in the Generation I games.

Genderless Pokémon
There are some Pokémon that have no Gender, usually because they are made of lifeless materials like metal. Some of there are Voltorb, Electrode, Beldum, Metang, Metagross, and Ditto. There are also no Legendary Pokémon with genders, except for Latias, Latios, Heatran and Cresselia.

Generation I
Articuno - Ditto - Electrode - Magnemite - Magneton - Mewtwo - Mew - Moltres - Porygon - Voltorb - Zapdos.

Generation II
Celebi - Entei - Ho-Oh - Lugia - Porygon2 - Raikou - Suicune - Unown.

Generation III
Baltoy - Beldum - Claydol - Deoxys - Groudon - Kyogre - Lunatone - Metang - Metagross - Solrock - Regice Regirock - Registeel

Generation IV
Arceus - Azelf - Bronzong - Bronzor - Darkrai - Dialga - Giratina - Magnezone - Manaphy - Mesprit - Palkia - Phione - PorygonZ - Regigigas - Rotom - Shaymin - Uxie.

Generation V
Genosekuto - Meloetta - Victini. (We are waiting for the english translation of the names, so we'll fix it when we have the translations).

Gender Differences
Starting in Generation IV, Pokémon in the Video Games sometimes had minor visual differences depending on their gender. Some differences were easily visible, while some are hard to see without looking closely.