I don't know anything about the ones for speedlights, my guess is that they work about as (in)effectively as other speedlight accessories.
On studio lights, they are, as the name suggests, a honecomb design. The light shoots through and is prevented from spreading out (much) because the light is forced through the holes, instead of just going everywhere.
They're used in a number of ways
1. To put a splash of light on a background (bright in the centre, darkening rapidly towards the edges
2. To light selectively, for example to light the hair only
3. As a backlight, so that the design stops spare light from striking the lens (but see note below on angles)
4. As a hard light within a soft light, sort of like a poor man's fresnel spot, placed in front of say a softbox to produce hard lighting on a face, at the same time producing soft lighting from the softbox
5. As a rimlight, illuminating just an edge
They are available in a range of different angles, from around 40 deg to 3 deg. I have them in 5, 10, 15, 20, 25, 30 & 40 deg but the popular ones are around 15-20 deg. The very narrow angle ones are mainly for commercial photography and I've only found them in The States.
Basically, with a 10 deg honeycomb fitted, the light can be pointing towards the camera without causing flare proided that it's pointing off at least 10 deg.