PSM is a platform for indie development on the PS Vita and PlayStation Certified Android devices. You code in C# using what it seems to be a heavily modified version of mono, with a rather minimal API to control some of the features in the PS Vita.
They also support publishing to their online store, with a yearly payment required.
So, to me, this looks very similar in idea and scope to Microsoft's XNA.
I haven't yet seen any PSM questions in here, and I would like to know if it would be okay to ask some.