להפסיק is the closest equivalent to Stop. לעצור originally means Brake, stop moving, but can often describe the halting of elaborate processes (while להפסיק is attached to immediate actions) - תפסיק לעבוד על התכנית הזו. - למה? - כי החלטנו לעצור את הפרויקט הזה לבינתיים. לעצור can also be loaned to other contexts to add color or dramatic effect. i don't think there's a definite answer as they often overlap, and there are some unique scenarios for both, i.e.- לעצור can also mean "to arrest" להפסיק is used for quitting a habit (הפסקתי לעשן for example) as a rule of thumb, if you can use "halt", then use לעצור. otherwise, use להפסיק.

it's "מישהו יכול להסביר לי" btw - "explain TO me"

Остановиться (самому) Остановить (кого-то) Stop (me) Stop (someone) =))