I found this excellent post of Zombie's when I was going through one of the forums thought I'd bring it back to life so to speak.
Quote:
The main differences between Fantasy and 40K...other than the old world and futuristic feel are as follows.
It's faster paced, and instead of artillery that sits in the back and firing, there are tanks and vehicles that serve the same purpose but can also act as transports.
Likewise, there are things called bikes and jetbikes, that act as fast cavalry, but usually can also shoot some sort of powerful weapon, and can move a bit further than fast cavalry.
Play works on average of 5-10 man squads, that can have some sort of Sergeant (Champion) or HQ (Hero,) that are attached that can pack some sort of punch. The other unique thing, is that depending on what army you are playing, and what army you are playing against, your squads can be specifically tailored to your needs. So you can add your own personal flare down to the smallest of units.
The biggest difference are the large array of weapons. Some are designed to punch through armo(u)r, some only work in close combat, some deny any type of save, there are things called flamers that work like breath weapon templates, that emanate from the model carrying the gun. They are perfect for clearing out bunkers. There are sniper rifles, and heavy weapons teams. There is just a large selection of things to chose from, depending upon your army.
Oh and instead of magic, you have what are called "Pyskers," who attack you through mental abilities. Instead of generating power dice, it goes off of a Leadership test.
The only other main difference too, are the armies.... I'll try to draw some comparisons to make points, but they are not as clear cut as this, so any hard core 40K players, don't grill me, I'm just trying to explain
Space Marines...kinda like Bretonnia...few of them but pack a punch
Imperial Guard--The Empire...weak foots soldiers but a lot of them, and the best tanks in the game
DaemonHunters/WitchHunters...If you really want to stretch it...Lizardmen...Very good pyskers, and tough ground troops, but few of them (Saurus and Temple Gaurd?)
Tau....In a broad sense Wood Elves, they are the best shooty army in the game
Orks...they are Orcs....'nuff said
Eldar....High Elves except better. You will loose all of your friends if you start an Eldar army.
Dark Eldar...kinda like dark elves, but they aren't very well developed, and you don't find a lot of DE players. I hear rumors of a new Codex coming out soon, so maybe that will boost interest.
Chaos Space Marines....All of the chaos armies rolled into one. You can take daemons (well you could...I don't know if that will change with the new Daemon's Codex, and the new rules edition coming out this summer)
Tyranids...I don't really know if there is a compare. These guys are just big bugs, that move fast and are nasty in close combat, and there are a lot of them. They lack a lot of fire power to make up for it
Necrons...The 40K Undead....I'd say more like Tomb King's because the squads are all pretty uniform and lack diversity. I play these guys, and they are fun, but they lack customizability, and after a number of years I've become bored with them, they don't keep me interested like the Vampires do. Though in 40K, you can actually field GODS with this army that are ridiculous, but are big point sinks. Also without getting to in depth into the rules, the neat thing about Necrons is that if they loose all their Wounds, you put them on their side, and at the start of your next turn on a 4+ they get back up. It's a little more complicated then that but that is the base rule. But they maybe getting rid of that with the new Codex.
I hope that helped and wasn't too long winded.
This is a very good guide to if you want to start a 40K army,but you currently play Fantasy.