A WordPress website, in its most basic form, is a platform where you can add content, manage it in all sort of ways, and present it to people online- a content Management system (CMS). It is plug-and-play in nature- with most features on the front and back-end being controlled by WordPress’s own user-interface, a theme and any number of plugins.
Before going into details, it necessary that you understand some basic terms (assuming that you are a complete beginner):
- Front-end: Everything that a visitor who comes to your website sees
- Back-end: An hidden workplace where the administrator adds and manages content, themes, and plugins.
- Domain: The address of your website usually preceding a .com, .in, .org and the likes
- Hosting: Think of it as digital real estate, providing storage space to build your website. Bigger or higher quality hosting will let you have more people simultaneously on your website but it will also cost more.
- Theme: A customizable pre-built template that makes the bulk of the website’s front-end
- Plugins: Similar to mobile apps- adds or upgrades functionality of your website.
- SEO: Search Engine Optimisation- Gets your websites on the front page of Google and other search engines
WordPress.com vs WordPress.org
WordPress.com is the company’s own hosted-platform. Depending on the plan you choose, WordPress gives 3-200 GB of storage, hosting, a variety of themes, plugins, security and backup options. Everything will be compliant with the ‘proper’ WordPress ecosystem and no AdSense ads can be shown. This is perfect if you only need limited customization and monetization options, in favor of a reliable and robust place to host content.
WordPress.org, on the other hand, is completely open-source. You don’t have to pay a single penny to download the platform and is even guaranteed updates for a lifetime! It is highly customizable with a multitude of free and premium, themes and plugins to your disposal. You can host any content and monetize from any Ad platform. However, if you break it you will have to fix it- that necessitates a constant check on security, hosting and a few other variables. Thankfully, there are plugins that do that for you too.
wordpress.org 
wordpress.com
Is WordPress Free?
As I mentioned, WordPress as a CMS platform is free for anyone to download. However, you still have to buy a domain and hosting, and these will make the bulk of your (small) initial investment. Most hosting providers offer monthly billing while the domain is rented for a year (minimum).
You can buy premium themes and plugins too, which adds to the cost.