Scheduled Task is just a Windows thing and PowerShell can control it in the same way it (PowerShell) can control many other Windows things.
Scheduled Job is, in the other hand, mostly PowerShell thing. Namely, scheduled job is some PowerShell code scheduled for execution in a "Windows Task Scheduler" manner and executed within PowerShell environment (naturally).
So, an activity run as a Scheduled Task is anything you can run from command prompt (eg. .EXE, .BAT, .COM, .CMD etc. - nothing PowerShell-ish, unless you decide to start powershell.exe and give it some script name to execute. Yet, you can control all of that from PowerShell.
On the other hand, Scheduled Job is exclusively PowerShell code (be it script block or script file) that PowerShell registers with Windows Task Scheduler, and Windows Task Scheduler knows how to activate it. Method of repetitive activation is in both cases the same ("triggers").
So, Powershell Scheduled Task is a Windows thing (controllable also from PowerShell), while PowerShell Scheduled Job is a exclusively PowerShell thing.