An optical tweezer is a device which uses light to impart forces in the fN to pN range. The basic setup involves focussing a laser beam (typically several 10s of mW of power at the output) to a diffraction limited spot with a high numerical aperture optical element. In the focal plane, the laser spot is highly localized and has a high gradient in intensity (ie. as one travels away from the spot centre, the light intensity drops off rapidly). In the dipole limit (ie. for small particles) it turns out that the force felt by an object is proportional to the gradient of the intensity. Since the intensity gradient is always pointing towards the centre of the beam spot, the particle will always be pushed towards the centre of the beam (read "trap"). The optical force also has a strong dependence on the size of the particle, which in general is not within the dipolar limit. Typically optical tweezers operate on colloids in the range of 0.5-5 microns. The full-blown analysis of the optical forces in this intermediate regime is quite nasty as it requires one to deal with Mie scattering theory (working with arbitrary orders of spherical harmonics to describe scattered fields).
Optical tweezers are commonly used in biophysics experiments as a handle with which to impart biologically relevant forces, for instance to unzip DNA and study the time-dependent forces that arise. They have also been used in non-biological settings to study colloid interactions in various fluid environments.