Starship in Star Trek. First seen in Star Trek: Deep Space 9, also seen getting whomped on by the Borg in Star Trek: First Contact.

This ship was designed by the Federation in response to the Borg threat. No working prototype was complete in time for the Battle of Wolf 359. The first ship, the USS Defiant, was completed one year after that battle. Benjamin Sisko was on the design team for the ship; that, and the threat of the Founders from the Gamma quadrant led to the first Defiant-class ship (the Defiant) to be stationed at Deep Space 9. The orignal Defiant was destroyed in a battle with the Breen; it was replaced by the San Paulo, which was recomissioned as the Defiant.

The Defiant class is built for war. It is very small, with a crew of 10 to 12; it can operate with a skeleton crew of 4. It has four photon cannons, ablative armor, quantum torpedoes, and is the fastest sublight ship constructed up to this point. Additionally, it has a cloaking device courtesey of the Romulans.

Possibly the only Federation starship class designed specifically for combat, not exploration.
The four Defiant Class Starships are:

1. NX-74205 - USS Defiant: Prototype and class ship; stationed at Deep Space Nine; destroyed by the Breen in an attempt to recapture Chin'Toka system (The Changing Face of Evil). Commanded by Captain Benjamin Sisko.
2. NCC-74210 - USS Valiant: Destroyed by an unknown class of Jem Hadar vessel (Valiant). Commanded by Cadet Tim Watters after the original captain and all senior officers died.
3. NCC-75433 - USS Defiant: Originally USS San Paulo; Recommissioned as USS Defiant after the destruction the original USS Defiant (NX-74205). Commanded by Captain Benjamin Sisko, and later Commander Kira Nerys.
4. Unknown: Never seen or mentioned on the show, to the best of my knowledge.

Note: In an early episode of Deep Space Nine, it was stated that The USS Defiant was a Valiant Class Starship but since then it has been named Defiant Class numerous times.

Some information taken from the Starfleet Ship Registry Database: http://www.webzone.net/rowan/sb907/ssrd/

The Defiant class is designated as an escort ship. It is almost entirely covered in ablative armour and carries 4 pulse phaser canons and between 2 and 5 torpedo launchers (I have looked at several schematics and seen them on either side of the aft impulse cowling(2), inside the forward warp cowlings(2), and below the main deflector(1)). The class started as a high warp torpedo ship with some exchange of design with the Sovereign Class Starship design team. It later became an escort and attack ship. It has three small shuttle bays and planetary landing gear similar to the much larger Intrepid Class Starship. The forward modual, containing an airlock, torpedo magazine, torpedo launcher and main navigational deflector, is detachable and fitted with a small impulse system so it can act as a single massive warhead for use in emergencies.