"For now" implies that something will be done later or at least not at this moment. "Now" means something is to be done at that very moment.

"for now" shows duration

To expand on Candi's answer a bit "for now" can also mean that something is good enough for the moment but that someone will have to come back at a later date and finish or improve on what they've done. Example: "The duct tape repairs on those pipes are good enough for now." The implication is that the repairs are okay for the moment, but a more permanent solution will have to be put in place eventually. Contrast that to Candi's definition which would fit more in a sentence like "We can hold off on doing this for now." Again it means that something is being put off until later. Now always means now. Right at this moment. Immediately. 今。