These are new to me, can anybody give a good description of their use or direct me to a link explaining their use for weathering?

Usually the final stage in weathering is to use pastels to add dirt. All previous work must be finished and if you are going to put your model into a diorama it is best to complete this as well. Pastels are not permanent so will rub off if the model is handled.
The pastels to buy are the chalky type. Oil pastels are no use for modelling. It is often possible to buy a set of about 12 pastels for a couple of bucks. If buying loose sticks, think of the use that they are to be put to and buy appropriate colours. Black, grey, white and brown are normally enough.
They work on a matt surface so if you have gloss painted your model you need to give it a coat of matt varnish before you start.
To use them, take a piece of fine sandpaper and rub the pastel onto it. You will get a small pile of fine dust on the sandpaper (Don't sneeze or you will cover everything with it).
With an old brush, pick up a small amount of the pastel and either dab it on or wipe the brush along the area to be covered. Don't be surprised to find it all falls straight back off. Pick it up with the brush and work it into the model. Start by wiping it back from exhausts, guns, etc. Blow off any excess, or dust with a large brush.
The effect is very subtle and it can appear that nothing is happening. Compare the area you have just worked with an untreated area and you will see the effect. If it all goes horribly wrong you can wash the model under the tap and start again when it has dried.
When you are confident in this area you can apply the pastel in other areas. A light grey pastel can make a convincing coat of dust and brown can look like dirt collecting round steps, etc.
I have always been wary of trying to fix pastels with varnish as it can make the effects disappear, I have had success by using a light spray with hair spray.
Another effect that can be achieved with pastels is to mix some brown pastel with water and treat it like a wash. It collects in puddles and looks as if the model has got muddy but some has been washed off by rain.

Hi Aaron-

I just responded to your post in the Armor forum about this technique, so you can look there to see if they create the effect you are after. I just recently started using pastels and love the results.

Basically - I just use these colors: Brown, Light Brown, Tan, Gray, Black, and White. I take the stick of chalk and scribble a bit on some sandpaper. This grinds it into a fine powder. Then i use an old dry paint brush to "scrub" a little bit of chalk on the model. Use very small amounts of chalk on the brush - kinda like you are dry brushing with oils. At first it wil be hard to see if it makes any difference - that is good.. you want to do a little at a time and "build" up the color. Take breaks and look at the model from a distance to get the proper perspective. When I first started out I would over do it all the time.... I couldn't see any difference right away, so I'd add some more chalk... before I knew it the thing was "painted".

Hope that helps-
Jeff

You can also drag black back from exhaust stacks in the airflow to create exhause stains. Reddish brown an make convincing rust streaks. Always start from the point where the stain originates and drag the pastel away. The straking will creat a natural fade, just like these effects have in real life. Using small. amounts of black on a face give an unshaven look and earth tones on feet and the base of unforms or vehicles make them look dusty. When doing this, start at the bottom and decerase the amount as you get further from the ground.