Linaro is not a distribution. It's a joint effort to *help linux run on ARM hardware*.
For example there are about three linux based distros for the openmoko, all with custom UI front ends.
And for these "linux based distros" to work you need a running linux (kernel) on which to base them. That's the work of the software company named Linaro.
To give into more details:
as a point of comparison, in the x86 processor world, things are rather standardized, and well modularized. There's more or less only one single main platform (the PC) with some weirder variant (BIOS vs. (U)EFI, or even weirder PC vs. custom Apple Intel Macs) and a few exception (they are really rare and don't matter much for mass consumption). With clearly organized components (no matter if you go for Intel, AMD, Via or more rare hardware: you've got the same basic CPU, northbridge, south bridge, PCIe bus, etc.). And well defined procedure to initialise and control everything. On the software side, all this is controlled by using clearly modularized and segregated components.
When something new arrive to the market (like the jump from BIOS to EFI, the move from PCI to PCIe, etc.) you only need to write a module and leverage what already exists for everything else.
To boot into linux, you use the same kernel everywhere, and only load different drivers depending on the local variations.
in the hardware arm world, things are much more messy: lots of weird SoC are put together, with far less standardisation. Also, whereas the x86 hardware is general purpose and widely available to everyone (just think about all the beige boxes everywhere. Then think that if you need server, a compute node or a home theaterPC you use the samecomponents. And branded machine (brand servers) use the same componenents too), lots of the ARM hardware tend to be put together for very specific custom usage (a company using their own SoC + PCB for a router. Another for a smart phone. Another for a NAS. Another for TV set top box. Etc.). Lots of this hardware is one-shot (there's few design re-use between a router and a smartphone).
as a consequence, before Linaro, Linux on ARM was a big mess: each time a company put together some hardware, they also do their own port in a one-shot fashion: they download the linux source, write a big monolithic patch to support their own weird variant, compile it, even sometimes publish the code (to be compliant with the GPL) and call it a day.
when another company wants another ARM-based machine, they just to the same.
No modularisation. No code re-use. No easy rebasing of the kernel.
That's why for several pieces of hardware, you're basically stuck with one very specific kernel version (openmoko is still at 2.6.x something) even when the source is available: the hardware depends on a huge monolithic platform drivers which is tighly dependant on the very specific kernel version against which the patch was written.
If there's any known kernel bug, your only hope is to wait for the back-port. you can't just move to a recent kernel (to 3.6.x).
If you want to provide a distribution for several pieces of hardware, you need one separate kernel per each separate device, sometimes different kernel version (depending against which kernel version was the patch written).
A big mess.
It's not a surprise that Microsoft is having big difficulties with Windows 8 on tablets and smartphones: the hardware landscape is really weird (and their own approach is to impose 1 single specific type of platform, so writing Windows 8 is easy and have everyone else standardize on it) (that's why they won't end up being as much popular on smart phone as they wished).
The role of Linaro was to put some order in this mess, by gathering together several of the people involved (there are hardware companies here) and giving them opportunities to work together and coordinate their effort. Among them as well as together with the kernel developers and the rest of the community.
Split everything into small component which will be easy to re-use (for newer slightly different hardware) or re-base (new kernel = keep as much of the drivers as possible). Organise everything cleanly.
The target: to achieve the same situation as with x86 hardware - same kernel everywhere, all the magic lies in modules and component. Re-using should be easy and cheap (just write new drivers for the new hardware elements, re-use as much as possible from the rest. Or even write a new batch of settings for a generic driver). Everything should be as modular as possible, and get everything possible back into the official kernel (no monolithic patches).
The end users see benefits (they get better quality of code, and longer maintainability. This requires open source code, but also require code which is easy to work with. The past mess wasn't. The future for which Linaro strives is).
The hardware makers see benefits (the initial cost of developing new embed hardware is lower, no need to write full monolithic patches, leverage as much as possible from what already exist. The overall development/maintenance is also simpler: no need to keep one separate kernel for every piece of hardware poduced, and easier to get updates from the mainline kernel).
And the kernel developing community see benefits (easy to keep and maintain modules in the mainline kernel, less headache when big changes arrive with newer kernel as everything is nicely modularized).
If today you start seeing interesting distros on ARM (lots of smartphone-specific distros. big name distros like Ubuntu and openSUSE. etc.) that's exactly because of the work behind the scene of developers like those from Linaro, making the Linux kernel easy to make work every where.