  Two types of radio emission are observed from X-ray binaries with jets. They
have completely different characteristics and are associated with different
kinds of ejections. One kind of emission has a flat or inverted spectrum
indicating optically thick self-absorbed synchrotron emission; the second kind
of emission corresponds to an optically thin "transient" outburst. The flat or
inverted spectrum covers the whole radio band and has been established also at
millimeter and infrared wavelengths. When this kind of radio emission is
spatially resolved it appears as a continuous jet, the so-called "steady jet".
In contrast, transient jets associated with optically thin events are resolved
as "plasmoids" moving at relativistic speeds away from the center of the
system. The most important point is that the two kinds of radio emission and
their corresponding types of ejections seem to be related to each other; the
optically thin outburst that characterizes the transient jet occurs after an
interval of emission with flat/inverted spectrum. Two different models
successfully describe the two jets: a conical flow and shocks. The conical
outflow describes the continuous jet and internal shocks in a continuous
pre-existing outflow describe the "plasmoids" of the transient jet. The
internal shocks in the outflow are thought to originate from a new population
of very fast particles. Three open issues are discussed: is magnetic
reconnection the physical process generating the new population of very fast
particles? Is that part of the continuous jet called "core" destroyed by the
transient jet and its associated shocks? Can we extrapolate these results from
steady and transient jets in X-ray binaries to radio loud AGNs?
