Devant versus en avant and derrière versus en arrière.  Devant and en avant are sometimes used interchangeably, as are derrière and en arrière.  What's the difference? One is stationary while the other is traveling.

Devant pronounced [d(əә)vɑ̃].  "De" sounds as if practicing the sound a D makes, "vant" sounds like v + the soft open A sound coming through both the nose and mouth at once (trying not to pronounce the "nt" because their presence indicates the nasal sound of the A)

En avant pronounced [ɑ̃ avɑ̃].  "En" sounds almost like "on" in English except that you try not to pronounce the n and you let the sound come out of both your nose and mouth.  The first letter A sound like a soft A in English, "vant" sounds like v + the soft open A sound coming through both the nose and mouth at once (trying not to pronounce the "nt" because their presence indicates the nasal sound of the A).

Derrière pronounced [dɛʀjɛʀ].  "Derr" sounds similar to "dare" in English but the r is pronounced in the back of mouth and with a more open mouth, "ière" sound like y+air with an open mouthed A and R.

En arrière pronounced [ɑ̃ aʀjɛʀ].  En" sounds almost like "on" in English except that you try not to pronounce the n and you let the sound come out of both your nose and mouth.  "Arr" sounds like the English word "are" but with a more open mouthed R, "i" in French sounds like "ee" in English, "ère" sound like y+air in English but the R is pronounced without closing and rounding the lips as we would in English. 

Devant means in front, meaning a movement where your supporting leg is planted and your working leg moves in front of your stationary body.  You could also use devant to refer to where a movement closes but not to a movement that travels forward.  Think assemblé going up and down in one spot but closing in front.  If that same assemblé were used to travel forward, you would use en avant.

En avant is used for movement that is traveling forward.  In other words, your entire body is moving towards the "front."  Sissone for example can be done en avant.

Derrière means in back (the opposite of devant), meaning a movement where your supporting leg is planted and your working leg moves behind your stationary body. Like devant, derrière can be used to describe where an action closes.

En arrière is used for movement that is traveling backward.  Your entire body is moving toward the "back."

I've put "front" and "back" in quotations when writing about en avant and en arrière because these are sometimes referred to as toward the audience and away from the audience, but I can also think of many instances where it more accurately means forward toward your own front wherever that might be or backwards traveling toward your own back.