An EFP is an Explosively Formed Projectile (also known as a 'Self Forging Fragment', 'Self Forging Warhead' or 'Explosively Formed Penetrator').
The concept of Explosively Formed Projectiles (EFPs) uses explosive energy to deform a metal plate into a coherent penetrator while accelerating it to velocities up to 8000m/s. Unlike traditional shaped charges, which tend to form long rod penetrators, an EFP will put most of its original mass into forming what appears to be a solid mass or slug (however its true form in some variants may also be likened to that of a hollow sheath).
The final shape and performance of the EFP will depend upon a number of parameters. These include the type and amount of explosive used, case confinement and material, liner shape, thickness, profile, material and peculiarities applied to the liner such as localised heat treatment or performance enhancing grooves. It can be seen from this list of parameter variables that an in-exhaustive variety of charges could be created and used for a similar number of applications.
The most prevalent applications are in bomblets (notably those contained in the L20A1 155MM artillery round) and certain anti tank weapons, notably Javelin, which uses the capbility to good effect in either the Direct Fire or Top Attack modes.
Due to the extreme velocities involved, the metal appears to flow like a fluid. In addition, the velocity at the tip of the metal flow is much greater than that at the rear of the jet, as more and more metal is acted upon by the explosive charge. The resulting velocity gradient causes the jet to break up into fragments, each of which acts independently on the target: i.e. it hammers its way through.
Target effects include a super-bright kinetic flash (somewhat similar to a very powerful flash bulb) and extreme velocity ricochet effects, including spall lining dispersal. Significant over-pressure effects are also present. It is fair to say that you don't want to be caught by one.