Cap, soft cap, or capture can refer to several different things depending on the context.
Relative to battlegrounds or world PvP, "cap" may be short for "capture," the capturing of an objective or object, such as a graveyard, flag, or tower. Many of these objectives require being physically held, such as a flag, or maintained dominance, in the case of a tower.
"Cap" may also refer to the level cap, or the highest level that can be currently obtained. During the release of World of Warcraft, the level cap was 60. The level cap has been raised for each successive expansion. In The Burning Crusade it was increased to 70, and in Wrath of the Lich King it was further increased to 80.
Finally, "cap" or sometimes "soft cap" may be used when referring to gear itemization. For example, players theorycraft about the hit cap, the expertise cap, the dodge cap, the spell penetration cap, and others.
A "hard cap" is a point beyond which a stat becomes useless. For example, 1400 armor penetration rating bypasses 100% of all armor, so more rating has no effect. A spellcaster with 17% increased chance to hit will never miss a boss, so this is the hard cap for hit rating for casters.
A "soft cap" is either the point at which a stat stops providing a particular benefit but continues to provide others, or the point beyond which a temporarily active buff will push the stat over a hard cap. For an example of the first, a defense rating of 689 is sufficient to eliminate the chance of a boss getting a critical hit against you, so further defense will not have any effect on your chance to be critically hit, but will still increase your chance to parry, dodge, block and be missed by attacks. For an example of the second, a character with a 
[Needle-Encrusted Scorpion] will occasionally get 678 bonus armor penetration rating. If the character has 732 base rating, this will be sufficient to temporarily reach the hard cap of 1400 noted above. Further base rating will cause some of the Scorpion's bonus to be wasted. However, the additional rating will still provide a benefit when the Scorpion's proc is not active.