How to play a Tank Destroyer in World of Tanks?

Many tank destroyers, and particularly the Stug III have a great camo rating, which means that if you play carefully the enemy team will have great difficulty spotting you.

While playing this particular tank, and other TDs like it, try to stay behind the rest of your team and lay down supporting fire form a sniping position.

If you're not familiar with how camo and spotting works, I recommend this camouflage tutorial.

Some general tips:

• Stay behind a bush, but remember that firing your gun will temporarily remove the additional camo value of the bush
• Equipping your TD with a camo net, and training the crew in camouflage will greatly increase your overall camo rating.
• Remember that moving your tank, even by slightly turning it lowers your camo value. With TDs and SPGs you can prevent accidentally turning your tank by locking it in place by using the (default) X key.
• The commander's sixth sense skill is great on TDs as it will help you know when you're spotted.
• The Stug III is responsible fast and mobile, if you are spotted or think you might be soon, relocate to a new position from which you can safely snipe.

As an alternate answer to the excellent camo-oriented answer @Xenox provided, there are also tank destroyers that do not rely on camo:

• British TD starting from the AT range
• German TD starting from Jagdpanther 2/Ferdinand
• US TD starting from T28/T28 Prot

Generally there are 3 types of tank destroyers:

• Camo-oriented: They are usually low profile, and turret-less
• Maneuverable: Either with turret, or very good turning speed
• Heavy armoured: slow, above average frontal armour

The camo one, @Xenox has answered.

For maneauvrable TDs (e.g. E-25, German turreted TD line...etc.) It's all about positioning, dishing out as much damage as you can, then withdraw and re-position, rinse and repeat. So learning how, and when to flank, learning paths to take so you can quickly retreat and move somewhere else, is the most important aspect for these kind of TDs.

Heavy armoured TDs (JP-E100, T110E4, T95...etc) is about finding the spots that conceal your weakpoints, avoiding being flanked, whether by positioning, or knowing where your team mates are and how they can protect your flanks. Knowing when to push forward is also very important due to the slow movement speeds of these tanks, not being at the scene of an important skirmish can determine a match's outcome. As a heavily armoured TD you can also take the role of drawing attention, since your shots hurt alot people tend to want to take down TDs as quick as possible, so if you can utilise your armour well, you can, for example, draw attention away from your flanking team mates.

There are numerous ways to play a TD, so it's not restricted to what I said above, e.g. a heavy TD can also play camo-oriented, if they can find bushes that are big enough.