Can i copy a slide from another presentation or is it plagiarism? For my seminar work i want to use a slide which i found in a presentation available on the internet. Now i wonder whether it is considered plagiarism if i copy the slide into my presentation while citing it.

If copying is okay, do I have to quote it on the slide itself or in the references at the end?
Standards for attribution are a lot lower in presentations than in formal papers. There is simply not enough time and space for it. It is unlikely that you will be accused of plagiarism for copying slides.

However, to be safe, you could add a small note to the slide or verbally say something stating where you got it from. Perhaps the creator put a lot of effort into the slides and wants to be credited.

Standards vary from field to field and country to country, so ask someone in your department to be sure.