Long story short...

Squarespace is a website builder and has many built-in features making it more beginner-friendly. No coding required although it is possible to customize by adding custom code if you have some coding skills (or know someone who does ;). Hosting is included with Squarespace pricing.

WordPress is a CMS (Content Management System) that is better suited for content-heavy sites with a lot of data to manage (think online store with lots of products). Some features are built in but more can be added through the use of plug-ins making it endlessly customizable. WordPress is a free and open platform, but other costs will be incurred as you may have to pay for templates and will have to get your own hosting and pay for that separately.

Click below to read a more detailed article on the differences.