Custom loops

Hello,

This might seem like a bit of a strange question but there’s something that I’m finding really hard to understand so was hoping someone could clear it up for me.

If I create a separate instance of the WP_Query object for a custom loop to work along side my main loop, lets say this custom loop is the first thing on the page, I then need to reset the loop so because my custom loop will have affected it?

Is that correct? The thing I’m finding hard to understand is that if I’ve created a separate instance of the WP_Query object then how is this affecting the main default page loop? Is it because the $post variable gets altered?

Thanks very much for any help, I’ve been searching for ages on the Internet for an answer and non of the ones I’ve found (of which there are quite a few) are really clearing this up for me.