Even more info, if you can stand it:) :
There are 3 basic types of needles-- straights (which you’ve been using so far), circular, and dpns, which stands for Double Pointed Needles.
There are 2 types of sizing-- the width of the needle, how thick it is, and the length. The first, the thickness, can be as tiny as .25mm or even less (used for tiny, tiny lace or doll cloths. Actually, the woman who did the clothing for the new Coraline movie used needles that size), and go up to 25 mm, which are the thickness of your big toe! They can be used for very thick yarn or very loose sts (I think they are very awkward to use). They are also sold by sizes in the US (they used to be sold by sizes in the UK, but that is rare now-- and the US and UK sizes were backwards of each other:???:). The US sizes go from 8 X 0 for the very thinnest, up to 25 for the very thickest. But here the most common sizes are
Socks-- from US 0 (2mm) - US 3 (3.25mm)
Worsted weight, which is probably what you’ve been using-- US 5 (3.75mm) - US 9 (5.5)
Bulkier weight, such as that used for Norwegian sweaters-- US 10 (6mm) - US 10.5 (6.5mm) and higher.
By the way, the US sizes are weird in that the 10.5 is 6.5mm and then there is a bizarre jump to US 11 which is 8mm. Makes no sense. But you can find 7mm and 7.5mm if you look around.
The thickness of the needle is very important because it will determine how big your stitches are, and how many sts you get per inch is call the “gauge”. But with a blanket or scarf, it isn’t nearly as important-- although if you are very off the gauge, it will come out way to small or large and if too large you could potentially run out of yarn (they might recommend 3 skeins and you end up needing 4 kind of thing).
The other needle sizing is the length, which will be given in inches or cm. For that, you use what is comfortable for you, or, in the case of your current project, what fits onto the needles.
Straights. These, as you know, have little knobs at the end so that the stitches won’t fall off. You can only knit flat on these (there is a technique for knitting tubes on it, but it’s extremely rarely used and we won’t worry about it here).
Circulars. These are 2 short needles attached to each other by a cord. You can knit tubes on them. OR you can knit flat-- just knit as if they were 2 separate straight needles. If you’re knitting something flat very long, they are very useful because you have so much length of cord to hold all of those stitches.
DPNs. These are as if you took a circular and chopped it into 3 or 4 sections. They are also for knitting tubes. There are videos on here to demonstrate how they work.
And you did fine posting your question! As you work on your blanket, if you have any others, post away!
