Newbie has some terminology questions...

Welcome!

A "wash" is a diluted mixture of paint, used mostly for 'weathering', to make something look worn. It is highly thinned down paint  (black, brown, etc) to give an effects of, for example, grease or grime on an aircraft. Or dust on an armored vehicle. Most washes are applied with an airbrush, though they can be brush painted on.

"Filler", as I know it anyway, is putty or perhaps some styrene strips or bits of sprue to fix gaps in parts that don't fit as well as they should, etc.

panzerpilot is correct about the filler.

A bit more info on washes.  He is correct about the use of washes that they are for weathering.  But they are also used to highlight details to give depth and dimension to a model.  Even on a very clean "factory fresh" sort of model that has little to no weathering.  

Washes are applied with a brush. If mixed to the proper consistency and applied with an AB it would just be another coat of paint, more or less.   You will likely hear the terms sludge wash and point wash.  A sludge wash is usually a general wash that will be applied over a broad area and can be for a variety of reasons.  General weathering for dust or grime etc, highlight some detail, changing the tonality of colors are a few.  It can be applied and dabbed with a tissue or q-tip for the desired effect.  Pin washes are more precisely applied, such as with a pointed brush, around rivets, at the base of a turret, the wiring in a landing gear bay etc to highlight the detail and make it look more 3 dimensional.

When starting out with washes, stay away from black.  The color of black paint is really much blacker than most of what is seen in real life. There really is very little around you that are that “black”.  Dark grays and dark browns are better suited for pin washes.  Since a sludge wash cover a bigger area and can change and overall color, it can be lots of different color and will be determined by the effect you’re after.

Filler is also used to change the contour of a feature.  Say there is a raised bump on wing or fuselage, but it is not as long as it should be.  Adding filler/putty to front, rear, or both allows reshaping it to a longer part.  This is only one example.  It can be used to increase size of wing or tail fillets, or  lots of features that are incorrect on a model.

"When starting out with washes, stay away from black." 

I use black, mixed with brown, in a very thin wash inside wheelwells, particularly WWII luftwaffe, where the wheelwells are RLM 02 grey. This represents the hydraulic-oil mix that can get in there. Other than that, I agree. brown, red brown or a dark grey is better on surfaces.

panzerpilot

"When starting out with washes, stay away from black." 

There's always an exception.

I use Flory washes for bringing out panel lines and other recessed detail on my aircraft. I usually use Dark Dirt, which is a dark brown/gray and usually does splendidly. 

But when I put it on the blue nose of my P-51 recently, it stood out as very, very brown. So in that instance I did have to go back and use straight up black. Rest of the aircraft stuck with the Dark Dirt, however.

This is a big website, JailCop..

Here's a good place to start, in the "How To" section of the website.

http://www.finescale.com//home/how%20to/glossary.aspx

 If you subcribe to FSM, you can also use this part of the site, and washes are covered in the videos..

http://www.finescale.com//home/videos/how%20to.aspx