What is the difference between systemd's 'oneshot' and 'simple' service types?

The Type=oneshot service unit:

• blocks on a start operation until the first process exits, and its state will be reported as "activating";

• once the first process exits, transitions from "activating" straight to "inactive", unless RemainAfterExit=true is set (in which case it becomes "active" with no processes!);

• may have any number (0 or more) of ExecStart= directives which will be executed sequentially (waiting for each started process to exit before starting the next one);

• may leave out ExecStart= but have ExecStop= (useful together with RemainAfterExit=true for arranging things to run on system shutdown).

The Type=simple service unit:

• does not block on a start operation (i. e. becomes "active" immediately after forking off the first process, even if it is still initializing!);

• once the first process exits, transitions from "active" to "inactive" (there is no RemainAfterExit= option);

• is generally discouraged because there is no way to distinguish situations like "exited on start because of a configuration error" from "crashed after 500ms of runtime" and suchlike.

Both Type=oneshot and Type=simple units:

• ignore any children of the first process, so do not use these modes with forking processes (note: you may use Type=oneshot with KillMode=none, but only do this if you know what you are doing).

From systemd's point of view, Type=simple is kind of fire and forget. Systemd just forks a process defined in ExecStart= and goes on its way, even if the process fails to start.