Wikipedia
htop is an interactive system-monitor process-viewer. It is designed as an alternative to the Unix program top. It shows a frequently updated list of the processes running on a computer, normally ordered by the amount of CPU usage. Unlike top, htop provides a full list of processes running, instead of the top resource-consuming processes. Htop uses color and gives visual information about processor, swap and memory status.
Users often deploy htop in cases where Unix top does not provide enough information about the system's processes. htop is also popularly used interactively as a system monitor. Compared to top, it provides a more convenient, cursor-controlled interface for sending signals to processes.
htop is written in the C programming language using the ncurses library. Its name is derived from the original author's first name, as a nod to pinfo, an info-replacement program that does the same.
Because system monitoring interfaces are not standardized among Unix-like operating systems, much of htop's code must be rewritten for each operating system.