Why Orks Rock in Warhammer 40k
Orks rock in 40k for a number of reasons. They are fun to play, for one thing, and you can beat most any other race with them if you know what you're doing. In terms of lore, they're probably the funniest, least serious of the races, and that can be a breath of fresh air in the otherwise dark 40k universe. Orks are basically a "horde" style race, so they are most effective when you have a lot of them on the field at once, and this can be easily done because most of the Ork units are cheap and come in large squads. For hobbyists, they can be fun to build and paint because most of their gear is "recycled" from other races, making for a very "mad max" feel to their equipment.
Lore
Orks are the British trailer trash of space. They speak in pidgin cockney, and have a crude hierarchy of slaves (runts), ordinary Orks (the mob), ruling Orks (Nobs, Warbosses), and special Orks (Weirdboyz, Mad Dokz, etc.) Orks have a psychic connection to other Orks called the "Waagh!!" that allows them to know who-is-who in the Ork hierarchy. It also spurs them on to commit acts of violence, often snowballing into large scale fighting, which has been known to wipe out entire planets. When other life forms in the vicinity have been destroyed, Orks will either try to find more life forms, or just fight amongst themselves
Strategy
Good Ork strategy relies on melee and large numbers. Most Ork units get bonuses to their melee if they attack en masse. It's a good idea to equip your units with choppaz for the bonuses to their close-combat attack. They can use firepower (dakka dakka), as well as psychic units and mechanized units, the same as other races, though more "orky." Usually a good rule of thumb with Orks is quantity over quality -- Orks units are so cheap, its often better to have a lot of them to overwhelm the enemy than to spend all your points on a few powerful ones.
The Figures
Ork units are fun to paint. They're fun to put together as well. The lore behind Orks is that they assemble their equipment from a hodge-podge of other equipment, so you can go to town customizing your vehicles to make them look however want.
