In general a wiggle is a sequence of small dipole magnets with alternating magnetic field (typically +-+-+- etc). The simplest wigglers have a sequence of N alternating + and - poles.
The synchrotron radiation (SR) induced by the wiggler adds to the total SR energy loss of the ring.
Wigglers are employed to increase the damping and lengthen bunches (for example at injection energy) in order to stabilize the beams. They can also be employed to control the emittance (mainly the horizontal one) of the beam by providing a controlled emittance increase. This may be useful in the presence of beam-beam effects to control the blow and therefore the beam-beam parameter of the beams.
A more sublte way to use the wiggler is by using for example a field B0 in two outer dipoles and a field 2B0 in the inner magnet. The total length of the 2 magnets with field B0 and of the magnet with field 2B0 is the same. In that case there is no net deflection as in any other wiggler. With such an asymmetric arrangement it is possible to speed up the build-up of polarization. The price to pay is increased energy spread (can be a problem) and a lower asymptotic polarization level.