What is shroud?Edit
Creatures with the Shroud ability cannot be targeted by instants, sorceries, enchantments, auras, or abilities (including yours). Any of the previous mentioned spells that were on a creature prior to shroud being applied are not affected.
Shroud can however be affected by a mass effect or any ability that does not target. For example deathtouch and wither, which are static abilities, do not target any permanent. So shroud cannot stop them. Spells that affect all creatures, such as Wrath of God, Damnation, and Day of Judgment, still affect shrouded creatures because they are not targeting any specific card. One card, Clone, can be a copy of Mist Leopard because it is not targeting it, even though it is becoming a copy of it. The card does not say "target" in its rules text.
Creatures with shroud can also be effected by spells that target a player or opponent, such as, Sleep
Shroud is one of the abilities that's good and bad. For example, say Mist Leopard. It is a 3/2 shroud. Goblin Artillery can deal 2 damage to target creature or player, which means Goblin Artillery could ordinarily kill Mist Leopard. But this is an ability, and since Mist Leopard cannot be the target of abilities, Goblin Artillery cannot target it, and therefore cannot destroy it in this manner. At the same time, Mist Leopard can't be the target of your own buffs, so it will likely stay a 3/2 creature unless some environmental, non-targeted effect is in play.
Shroud has technically existed for a very long time in the game, though it only got named as a mechanic in Future Sight. Green is currently the biggest user of shroud, but blue has used shroud significantly in the past and white has been known to offer shroud, as well, but more in activated abilities like with Sighted-Caste Sorcerer.
This creature cannot be the target of spells or abilities your opponents control.
This ability is known as hexproof, and was keyworded in 2012 Core Set. Before this it was known as Troll Shroud, because creatures like Troll Ascetic and Thrun, the Last Troll sported it. It is much better than shroud because the creature can still be affected by spells and abilities you control.