They have the same meaning indeed, but their use is a matter of grammar. They basically all mean "since" (or "from"), but here's how you should use them : "Depuis" can be used with a noun directly after it, or a time etc.. "Depuis ce jour..." = "Since that day..." "Depuis que" allows it to be followed by a verb, in the past. "Depuis que je t'ai rencontré..." = "Since I met you..." In French, unlike in English, you can't put the verbal group directly after it. You have to use "que". "Depuis lors" paraphrases a moment we explained earlier. It means "since then" "Un jour, je me suis perdu en forêt. Depuis lors, je n'y vais plus." = "One day, I got lost in a forest. Since then, I don't go there anymore." I hope this is useful :)