  We review the present state of understanding of solid friction at low
velocities and for systems with negligibly small wear effects.
  We first analyze in detail the behavior of friction at interfaces between
wacroscopic hard rough solids, whose main dynamical features are well described
by the Rice-Ruina rate and state dependent constitutive law. We show that it
results from two combined effects : (i) the threshold rheology of
nanometer-thick junctions jammed under confinement into a soft glassy structure
(ii) geometric aging, i.e. slow growth of the real arrea of contact via
asperity creep interrupted by sliding.
  Closer analysis leads to identifying a second aging-rejuvenation process, at
work within the junctions themselves. We compare the effects of structural
aging at such multicontact, very highly confined, interfaces with those met
under different confinement levels, namely boundary lubricated contacts and
extended adhesive interfaces involving soft materials (hydrogels, elastomers).
This leads us to propose a classification of frictional junctions in terms of
the relative importance of jamming and adsoprtion-induced metastability.
