There are four kinds of SIM card, sized to fit different devices:
Standard SIM - A classic full-size SIM, and the biggest of all. Generally only used in older phones, including old feature phones.
Micro SIM - A mid-size SIM, smaller than a standard one. Various phones from last few years take this size, including the Samsung Galaxy S5 and iPhone 4.
Nano SIM - The smallest SIM of all, and usually the SIM of choice for modern smartphones. This is the one you need for an iPhone 5 or newer, Samsung Galaxy S6 or newer, or a Google Pixel, for instance.
Combi-SIM - A clever three-in-one SIM. You can change the size of this SIM to make it fit in any phone. The majority of networks offer these as standard.
To find out what size SIM you need to get, take a look at your device's manual, or check the manufacturer's website.
If you're still not sure, don't worry - most networks send out combi-SIMs to make life easier.