  Blazars are a class of active galactic nuclei which host relativistic jets
oriented close to the observer's line of sight. Blazars have very complex
variability properties. Flares, namely flux variations around the mean value
with a well-defined shape and duration, are one of the identifying properties
of the blazar phenomenon. Blazars are known to exhibit multi-wavelength flares,
but also "orphan" flares, namely flux changes that appear only in a specific
energy range. Various models, sometimes at odds with each other, have been
proposed to explain specific flares even for a single source, and cannot be
synthesized into a coherent picture. In this paper, we propose a unified model
for explaining orphan and multi-wavelength flares from blazars in a common
framework. We assume that the blazar emission during a flare consists of two
components: (i) a quasi-stable component that arises from the superposition of
numerous but comparatively weak dissipation zones along the jet, forming the
background (low-state) emission of the blazar, and (ii) a transient component,
which is responsible for the sudden enhancement of the blazar flux, forming at
a random distance along the jet by a strong energy dissipation event. Whether a
multi-wavelength or orphan flare is emitted depends on the distance from the
base of the jet where the dissipation occurs. Generally speaking, if the
dissipation occurs at a small/large distance from the supermassive black hole,
the inverse Compton/synchrotron radiation dominates and an orphan
gamma-ray/optical flare tends to appear. On the other hand, we may expect a
multi-wavelength flare if the dissipation occurs at an intermediate distance.
We show that the model can successfully describe the spectral energy
distribution of different flares from the flat spectrum radio quasar 3C 279 and
the BL Lac object PKS 2155-304.
