Close combat involves attacks made by opponents at very close range. Any weapon which has a range of “C” is a close combat weapon and may be used against adjacent models with no penalty.  It usually cannot make ranged attacks, unless stated on cards. For example, the Strain's reach ability enables a close combat attack two spaces from the intended target. 

As opponents are frequently adjacent whilst making a close combat attack, this will mean that they are engaged, and will incur a penalty for making ranged attacks against non-adjacent models. The attacker will roll 2d6 rather than 3d6 when resolving the attack. The use of a ranged attack against an engaged model ('firing close') will result in a modifier of [MOD] [DEF] 3.

A model making a ranged attack on an enemy that is engaged with allies of the attacker is referred to as 'firing into close combat.'  The target will receive [MOD] [DEF] 3 for the first adjacent attacker's ally and [MOD] [DEF] 1 for each additional attacker's ally.