Choosing zips can be a lot harder than you would think.
There are different types, different colours, different weights, different materials.
Firstly you can split them into concealed/invisible and visible/exposed.
When sewn into a garment you can either see the teeth or you can't. Visible when you can see the teeth, invisible when you can't.
If you use a concealed zip you won't be able to see it from the outside, whereas if you use a visible zip you will be able to see it so this will affect the look of your garment. You might want an obvious zip somewhere as a style feature, but if not concealed is the way to go.
Concealed zips mostly have plastic teeth and woven tape.
Concealed zips come in different weights which suit different fabrics. The heavier the fabric the heavier the zip you should use. The most light weight concealed zips have almost sheer tape so can be used with very lightweight fabrics. You need to choose the correct weight zip to fabric as a light weight zip on a heavy fabric is more likely to break from extra strain.
As you can't see a concealed zip from the outside its possible to use a longer length zip than you actually need and either leave the excess inside the garment or cut it off. You should also use a zip 2cm longer than the actual zip opening so you have excess to stitch down inside and to allow you to stitch it in close right to the bottom of the opening.
Concealed zips can be easy to put in and even more so if you have a specialised concealed zip foot for you machine. This foot holds back the teeth as you sew meaning it can stitch really closely to them so nothing is visible from the outside.
See picture across - the hole in the top is for the needle and then the piece at the front runs along and holds the teeth out of the way, and you can sew it on either side of the foot for each side of your zip.
Visible zips come in many forms.
Firstly they can be split into closed ended, open ended and double ended.
A closed ended zip only opens at the top and is connected together at the bottom, for example on a pair of trousers. An open ended zip completely comes apart, for example on the front of a coat or jacket. A double ended zip has 2 zip heads/pullers, so can be opened from the top and the bottom, you would also find this on a jacket/coat.
Visible zips can be made with either a plastic coil, metal teeth or plastic teeth, and come with many different size teeth and tape.
Again with these zips it is good to think about the weight of the fabric you're using against the weight of a zip. You might want to use a heavy zip in a very lightweight garment but it may put pressure on the fabric and cause it to rip.
The size of the zip is measured by the width of the teeth in mm when the zip is closed. This is often referred to as the zip gauge. As the numbers get higher the teeth get wider. So a zip that is referred to as a 5 Gauge or a #5 is a zip where the teeth (when closed) measure 5mm across. Its a simple as that!
Remember you can stitch your exposed zip either on the inside of the garment or have the tape on the outside as a feature. Like the dress opposite which has a printed snakeskin zip tape and wide zip pullers.
Also think about the colour of the zip you want to put in, if you want it to blend in and complement the garment or if you want it to be a bold feature or a pop of colour.
Exposed zips need to be bought in the correct length for the garment. Zips with metal teeth can be shortened but it is not easy to do, it requires cracking off the extra teeth and replacing the stopper, so its not advisable as it can easily go wrong. So, always be sure to measure your opening correctly before buying the zip. - You could also lengthen or shorten the pattern slightly to accommodate a shorter or longer zip that you want to us. The only time it would not be advisable to use a shorter zip is when it is going through the waist and finishing at the hip on a fitted garment/trousers, as you need the length of the zip to allow you to get in and out of the garment.
Exposed zips are sewn in with a half zip foot which often comes as standard with most domestic machines.
Hopefully that covers the basics for choosing the right zip for your project.