an edge dislocation
The edge dislocation is the easiest kind to picture. Take a neat stack of paper, all sheets running the full width — that is a perfect crystal, each sheet a plane of atoms. Now slide one extra half-sheet in from the top, going only halfway down before it stops. That extra half-plane has a bottom edge buried inside the stack, and that buried edge is the dislocation line. Everything above and around it is squeezed together to make room; everything just below is stretched a little apart.
So an edge dislocation is simply the edge of an extra half-plane of atoms wedged into the crystal, marked in textbooks with an upside-down T symbol (the vertical stroke is the half-plane, the horizontal stroke the slip plane it will glide along). Its defining feature is geometric: the Burgers vector — the amount and direction of slip it carries — points perpendicular to the dislocation line. When a shear stress is applied, the line moves sideways, in the same direction as the Burgers vector, by repeatedly handing the extra half-plane off to the next row of atoms. One bond breaks and one reforms at a time, which is why it glides so easily.
Walk the mechanism through: under shear the buried edge shifts one atomic spacing to the right by swapping which plane is the 'extra' one, then again, and again, until the half-plane reaches the surface and leaves a one-atom step — the crystal has slipped by one Burgers vector. Repeat with thousands of dislocations and you get the visible, permanent bending of a metal. Right at the line the lattice is distorted: compressed above the slip plane, in tension below, and that little strain field is what lets solute atoms and other obstacles grab hold of the dislocation and strengthen the metal.
In a transmission electron microscope an edge dislocation shows up as a dark line; the extra half-plane and its strain field bend the atomic rows visibly. When it finally exits a crystal it leaves a step exactly one Burgers vector high on the surface — direct proof that slip happened one dislocation at a time.
An extra half-plane wedged in; its buried edge is the line that glides.
An edge dislocation can also climb (move perpendicular to its glide plane) by absorbing or emitting vacancies, but this needs diffusion and is only significant at high temperature — it is the atomic basis of creep, not of room-temperature slip.