Starfleet's starship design philosophy changed radically after the unsuccessful attack on Earth by the Borg and Starfleet's defeat at Wolf 359 - it became readily apparent that starships designed to do absolutely everything (such as the Galaxy Class) were becoming obsolete. A shift was made towards designing starships with a narrower focus - the Defiant Class Starship was a natural outgrowth of this philosophy as it was designed from the ground up to be nothing more or less than a warship.
The Intrepid Class (of which the USS Voyager is a member) is a further advancement on this theme of specialization though in the opposite direction. Designed as a small, fast science vessel, Intrepid Class Starships filled the role left by the aging (and woefully under-powered) Oberth Class Starships by combining the latest in computer, sensor and warp engine technology with upgraded weapons and shielding systems. When not pursuing scientific studies ships of this class are regularly put to use as diplomatic couriers or as scout ships during times of war due to their high sustainable warp speed.
While still comprised of the standard design elements of federation starships (a primary hull, an engineering section, outboard warp nacelles, etc.) her lines are far more fluid than even the Galaxy Class. The connecting plane between the two main hull components has been eliminated and the primary hull has lost its circular nature, giving ships of this class the appearance of being stretched along the y-axis. In a way, Intrepid Class Starships are the most aquatic-looking starships to date, appearing to be far better suited to knifing through the sea than slicing through space.
This is also the only class to feature variable geometry warp nacelles; more on that in a minute.
The class is tiny for a starship - 15 decks and 350 meters in length with a crew of 150, it comes in at just under half the length of a Galaxy Class Starship with a third of that class' draught and with a seventh of its crew. Its maximum sustainable speed is warp 9.975. The class is defended by 13 Type X phaser arrays, 4 photon torpedo launchers (two fore, two aft) and a computer assisted shielding system that analyzes incoming directed energy fire and modulates the shields to be more effective. The class' computer systems were the first to utilize bio-neural gel packs, synthetic clusters of neurons spread throughout the ship to route information and analyze data more effectively and with more flexibility than a standard computer core, though they were used in conjunction with a computer instead of in the place of one. Interestingly enough, this technology opens the ship up to new headaches - the gel packs can become infected like any other biological system, causing all sorts of juicy problems.
Variable geometry warp nacelles are a design feature that supposedly reduces the damage that warp engines do subspace - when an Intrepid Class Starship goes to warp, its nacelles pivot up to a 45 degree angle. In reality, the people responsible for the class' design wanted something that would make Voyager more visually distinct from her brethren, and bits of it moving around in a dramatic manner seemed like a good way to go about that. Within the science of the series, the technology just doesn't make sense - seeing as how the engines don't do any damage when the ship is moving at impulse anyway, the nacelles should have just been built into their warp configuration. It does look pretty cool, though.
Also unique to a starship class of Intrepid's size is its ability to land on a planet's surface and, barring any complications, return to space again, giving the ship the ability to set itself up as a mobile science base-station without the hassle of constructing a separate one. Not that we'd ever seen it used that way - more often than not, Voyager would land to hide, not explore.