So I have been newly introduced to interchangeable needles, but there are somethings I still don’t understand. These interchangeable needles, exactly how are they interchangeable, is it changeable from circular double pointed and straight, or only some of the three. Also is it worth buying them, are they reliable? Can anyone explain to me what they are and if I should invest in them? Thanks 
Interchangeable means that you can change the needle tip size or the cable length. Not all circular needles are interchangeable. Some are fixed circulars. They can all be used for knitting flat or knitting in the round. 
There is a search link right above the ad. If you put in interchangeable needles you’ll got lots of info that has been posted about them if you are interested in them. 
You can use any circular the same as a straight needle, but on some of them the tips are permanently attached. The interchangeables are a set of circular needles with a couple different cord lengths and different sizes of tips that screw on to the cords.
i purchased my first set of interchangeable needles almost a year ago and i think they’re great. my set came with stoppers so i can use them as straights if i want and three different cable lengths so i have so many options. plus i have sizes 1 through 15. of course i will continue to expand my collection of needles but if i didn’t want to i would have close to anything i might need. i say go for it. you can never have too many needles.
Interchangeable needle sets are TOTALLY worth the investment. Do yourself a favor…get them as soon as the budget allows! I wish the variety of brands had been around years ago.
Many interchangeable needles come in sets. You get a range of needles head sizes and a number of different length cables. This is the most cost effective way to purchase interchangeable needles. However, if your budget does not allow this large of an outlay of funds, many sets allow the purchase of individual cables and individual needles. Start with the purchase of the needle and length of cable called for by the project you are working on now. As this nears the end you order the needle size needed for your next project. You can use the cable you already have, if long enough, or purchase another cable if needed.
As you need a needle buy another needle. The individual cost plus shipping will be higher than the whole set. But then you only have to purchase the needles you will be using. I have used interchangeables for over two years and I have yet to use the larger needle sizes.
I have a set of Denise that I’m very happy with. The only drawback is that one of my connectors has evidently worn, so sometimes the needle pops off the cable if I put too much pressure on (as when sliding stitches).
I especially like to use the set for knitting gauges because I can easily switch needle sizes by just popping a new one on the cable. I also use a needle without a cable attached for binding off. I can drop a needle head into my project bag that’s out of the way until I need it to bind off, since I usually bind off with a larger size needle than I knit with.
The cables make good stitch holders, too - just pop a needle back on to resume knitting, rather that transfer back and forth to traditional holders.