Shimano SLX 10 Speed Dyna-Sys Groupset M660-10
Designed as the workhorse in the Shimano MTB range, the SLX groupset features a lot of the functionality and performance of the higher end Shimano Deore XT and XTR components, at a far better value pricepoint. With durability it's main drawcard, riders choose SLX for its bombproof performance and ability to stay tight and smooth even in the crummiest of conditions. For 2012, the SLX Drivetrain has been upgraded to Dyna-Sys technology to enjoy the many benefits of the 10 speed system, including a wider gear range, smoother shifting and more reliable front derailleur performance. Although pure weight weenies will still want to plump up for Shimano XTR or XT, SLX will appeal to those after raw function and performance.

From Shimano:

"Tough. Aggressive. Light. For the latest off road styles... The previous Deore LX, our mid-range MTB component range, has been split into two distinct groups. Deore LX remains in the family, but it's now purpose built for Trekking. SLX series takes over as the mid-range MTB group designed specifically for off-road riding. The distinct range of options within SLX allow you to tailor your components to your preferred off-road style, from XC through to All-Mountain". "" 

So what exactly have they improved on?
Well aside from the obvious inclusion of a 10 speed cassette, Shimano have made several key changes to imrpove shifting on your bike. The Dyna-Sys SLX Chain itself is now a asymmetric number, with one side that is made to work optimally with the shifting pins and ramps on the front cranks as you move into a bigger chainring, and the other side is made to work with the shifting ramps as it climbs up the cassette. This is particularly noticeable when shifting under load, as the 10 speed drivetrain is so much quicker and more accurate than previous generations.
The SLX 10 speed cassette has a larger spread of gears than the old version, with an 11-36t option giving you a super low gear for technical climbing and being particularly popular with bigger-wheeled 29er bikes. Shimano still have the 11-34t and 11-32t options which will be better for XC and trail riders, though all of the new cassettes feature ratios that are tighter than the previous 9 speed versions due to the extra cog.
Thanks to the new wide-range cassette out back, the SLX Crankset received an adjustment to the teeth ratios to make front shifting even better. Instead of the old 44/32/22 chainring spread, the new cranks use a 42/32/24 spread to reduce the gap between each chainring. On the new 10 speed drivetrain, the lowest and highest gears are almost identical to the 9 speed setup, though there are now simply more usable gears in between those extremes.

And what does this mean out on the trail?
Well the main benefit is that you'll reduce the amount of shifting up front, which most people will agree is a very good thing. With a bigger spread of gears at the back, you can spend more time using the middle chainring with less need to drop down into the granny, and you can spend more time focussing on the trail ahead. This is also advantageous on a dual suspension bike, as most duallies have their rear suspension designed to work optimally around a 32t middle chainring - if you spend more time in the middle ring, you'll have less suspension feedback and less suspension bob.
After spending some time on the new 10 Speed Groupsets, we can honestly say that it is smoother and more responsive than any 9 speed components from the past - which is exactly what you want from your drivetrain.

The Shimano SLX Groupset includes:

• Shimano SLX M660 Crankset with 24/32/42 triple chainring
• Includes SLX Bottom Bracket
• Shimano SLX Shifters
• Front SLX Derailleur (Hi Clamp, Low Clamp, E-Type or Direct Mount - check your bikes specifications) 
• Rear SLX Derailleur with Long Cage
• Shimano SLX 10 speed Chain
• Shimano SLX 10 speed Cassette (11-32t, 11-34t or 11-36t ratios available)

Shimano SLX components offer exceptional performance and durability at a great price