Segmentations are essentially a special type of organized list. You can use them to control dynamics as well as filter smart lists, campaign smart lists and flows. Basically they allow you to customize your customers experiences.
Examples of segmentations include: A geography segmentation that breaks people out by state. A Customer segmentation that breaks people out by new or existing customer or by number of employees.
You can use segmentations on emails and landing pages to control what people see what. For example you can make sure that if someone from AZ visits a landing page or gets an email the content is different than what someone in FL would see. You can change images, text blocks, forms or snippets.
Snippets are like an image in a digital billboard it changes constantly and can be pushed to multiple places. In a snippet you can use images & words to create a section of a landing page or email that changes when a certain segment is triggered. And if you change that snippet it changes everywhere that you placed it. So I could create one snippet for AZ and one for FL and tie them to the segments and put them on multiple landing pages and emails. Then later if I had to switch out an image or words I would just open up the snippet, make my changes and save it. It would automatically populate to everything I put it on.
Litterally one change multiple impact.
Here is a good deep dive that covers what you should need.
Personalization: Segments & Snippets