Developers can control what you record on the PS4
Not every part of every game will be recordable on the PlayStation 4. In a likely effort to curb spoiling key moments in certain games, developers ultimately have control over what you can and can't record while playing.
As revealed in the latest PS4 Ultimate FAQ, Sony states, "Users may not be able to record certain areas within the game. Developers can select the area of restriction, and what will be restricted will depend on game titles. In these cases, recording will be temporarily suspended during the scene."
Outside of these restricted areas, the PS4 will constantly record your gameplay footage automatically in the background. Up to a maximum of 15 minutes can be recorded before you press the Share button. You actually can't choose whether or not to record your gameplay, but the footage will not be exported and will not occupy space on the hard drive until you actually press the Share button.
As long as you have sufficient free storage, you will be able to store the captured gameplay footage and trim the video in the Share button interface, selecting the starting and ending point of the video. Unfortunately, you can't add voiceovers to a shared video clip, though you are able to speak in real time when streaming live video.