Sorry if this is a stupid question, but what exactly does 'yo' mean? I hear it on the end of sentences a lot but I'm not entirely sure what it means.

Does it act like 'ka', a spoken punctutation mark?

if you're talking about the ka at the end of sentences, it makes something a question...
the yo at the end is kind of has the nuance that you're letting someone know something.

Could you perhaps give me an example of when you would use it and when it isn't needed?

Explained here:
http://www.guidetojapanese.org/adgobi.html#part5

I like to think of it as similar to the English "Did you hear,...!" or "Guess what,....!"

そらはピンクいろです。
the sky is pink.(statement)

そらはピンクいろですか。
Is the sky pink? (question)

そらはあおいですよ
Actually the sky is blue, I tells you (idiot)

Excuse my hiragana. I'm guessing from your questions you've more or less just started learning. Good luck!

Thank you very much. You've clarified it for me.