Taken from the WoT forums
Not all TDs are created equal many have different roles and play styles
1) Heavy Tank Destroyers:
A majority of the TDs in this game fall unto this category for their high accuracy and penetrating guns with decent stealth and view ranges these play well as 2nd line fighters because of well angled armour and can also be used from stealth for good ambushes to take down the big boys.
Example's include T25-AT,Jagpanther, and SU-100
2) UCs:
These types of "TDs" are more of a Scout, shoot , and scoot vehicles.But, have light to no armoured and carry decent, but slow guns or beefy single shot howitzers.
Examples include the UC-2pdr, T82, and Alecto.
3)Assult Tanks:
This series of TDs is usually the most poorly played by many new players who try to play them as a long range sniper. With high armour but slow and little mobility , with either high powered cannons or quickfire high pen guns.While it can be effective at times its not what these were built for. These tanks are excellent front line wedge tanks for leading an assault with heavy and med tank support with them to prevent flanking or very good 2nd line support if bottom tier for the top tier heavies.
Examples include all AT-Series tanks , T28,and T95.
4)Bunker Busters:
While many other TDs fall into this category as well some TDs simply are these powerful guns and great for causing damage with a bang. These tanks have various armor arangements but commonly have insanely powerful cannons. That were designed to take out armoured buildings but can be devestating to tanks as well play with causion.
Examples include FV215b-183, SU-152, and ISU-152
5) Anti-Vehicle SPGs:
These are powerful rapid-fire support TD's that play as far back as possible. Two words to why its played like this. "No Armour". These are strictly sniping guns. When played right however can deal damage fast and hard. When played wrong you will be a flaming wreck in 1 or 2 shots play wisely.
Examples include the SU-76 and Nashorn