Processes
Every time a user issues a command, a program is run and one or more processes are generated.
Processes exist in a hierarchy. After the kernel is loaded in memory on boot, the first process is initiated which - in turn - starts other processes, which, again, can start other processes.
Each process has a unique identifier PID and a parent process identifier PPID.
Here are some commands to analyze processes.
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ps: Shows information about the executing processes as a snapshot.
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pstree: Shows the processes in a hierarchy.
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top: Shows information about the processes in real time.
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htop: Similar to top but it is a lot more modern.
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free: How much memory is being used and how much is available.
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uptime: How much time the system has been up and the load average of the last 1, 5 and 15min.
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pgrep: Process with the grep functionality.