Void Shields are the most ubiquitous 40k energy shield used by the Imperium.
They are comparatively huge to other Imperial shields. Traditional void shield generators are so large that only Titans and naval ships may carry them or they have to be mounted as defensive emplacements. There are special man-sized versions, although these are prohibitively rare and expensive. Classic void shields were pretty puny, and made up for it by being layered one under another so you had to take them all out to hit the thing they were guarding. "Modern" ones are beefier, can actually take a hit, and usually come in sets of 1-3. However, these "single shields" are actually arrays of smaller shields overlapping, much like the older fluff, and are best described in the novel Titanicus by Dan Abnett.
Void Shields displace missiles and projectiles by placing them into another dimension, which is why missiles go *Poof!* when they touch them. Void shields are highly durable, capable of withering punishing salvos from enemies and those large enough are capable of withstanding capital ship-grade weaponry. However, one of their main weaknesses is that they draw a large amount of power by just keeping them online and they can be disabled by simply shooting at them so much that the carrier's power source can't maintain the field.
The Imperium, being the Imperium, has also found a way to weaponize the damn things: get a ship with Void Shields close enough to another ship, or a space station, or a planet or what have you, and then turn those suckers on. Bam! Suddenly, your enemy's missing half of his shit and you didn't even have to fire a single gun.
Void shields used to suck in Epic, since absolutely any hit would take one down; they were basically your "that hit didn't count" hits before taking any body damage. Titan Legions altered this so that the attacker had to have at least a -1 saving throw modifier (ie, be at least a Lascannon), ending the spectacle of a bunch of infantry advancing on a Titan to fire a billion attack dices' worth of bolters at it before anyone actually fired any good weapons.
On the TabletopEdit
The biggest of fortifications, titans, and some land vehicles of the Imperium of Man have void shields, which are basically extra AV12 hullpoints which also shut down if hit by a Destroyer Weapon automatically; however, they come back on a 5+ at the end of your turn (which means, for protecting something which is already durable - as they are consistently described as doing - they are basically inferior to just building your hull out of storm shields, despite their allegedly much greater power draw).
Forgeworld, the crazy fucks, have decided that any independent character in a 30K army can buy one as a relic; it's a large blast centered on the user that acts like the aforementioned.
In 8th edition they act as a simple save against ranged attacks which can also block mortal wounds, starting at a 4+ for the war hound Titan up to a 3+ for the warlord (and presumably a 2+ for an emperor Titan). They do degrade as the model takes damage however, but it’s worth it for the added protection they provide assuming that your Titan doesn’t get into melee against another Titan, it’s times like this where the hierophant makes a mockery of void shields. As the void shield is a simple saving roll for the model it does not protect surrounding troops unlike old school shields, but this is the case almost across the board when 8th edition arrived thanks to the removal of templates.