Direct copyright infringement, as discussed in previous posts, consists of a personal trespass to a copyright owner's statutorily-granted rights. In certain situations, one can be secondarily liable for contributory or vicarious copyright infringement. Neither is expressly permitted in the Copyright Act, but neither is prohibited.
To be liable for contributory infringement, the defendant must contribute or enable an infringement by intentionally inducing or enabling another to engage in direct infringement. This would be the proverbial devil on one's shoulder, encouraging that person to infringe someone's copyright.
To be liable for vicarious infringement, the defendant must profit from an infringement while declining to stop or limit it. Implicit in this is the ability to control the infringing activity.