Factorio Forums

While the rest of the world enjoys 0.15.6 the Steam Linux version is still 0.14.23. Is this a Steam problem or Linux version generally lags behind?

All versions come out at the exact same time.
You should opt in "experimental version" for Factorio on Steam

Thank you, worked great!
What distinguishes experimental and regular versions and how to find out which one is experimental?

There's no "magic" way : you have to know it.
Basically, each major update comes out as experimental (0.14.0, 0.15.0, ...)
Then there's a lot of bugfixing, balancing, small changes that are made bu the devs, and regularly pushed out as minor updates. This is still considered experimental stuff. This phase lasts a few months.
Once the devs are satisfied with the content and stability of the last version, and thing it deserves the "stable" attribute (usually, it's meant to be the last update of its major version, but sometimes some late bug compells the devs to publish another "stable" release for a giver version), then they tag it as stable before pushing it.

If your Factorio on Steam is set on "download experimental stuff", you'll get all the small increments during bugfixing phase. New content now, but you'll likely encounter bugs, maybe gamebraking. This is still alpha.
If your Steam is set on "only stable", then you'll have to wait the devs are satisfied enough with the current version before Steam updates your client. You'll get the content a lot later, but it should be as bug free as it can be.
Your choice .

Factorio's webstie now show which version is stable and wich is still experimental : here

Factorio is an early access game thus all bugs are viewed as features and in game challenges. Experimental for maximum fun ;D