What is the difference free theme and genesis theme in wordpress

Genesis gives you the option to modify their themes to suit your needs or to purchase premium themes that are not so widely available. In contrast, millions of people use the free WP themes that turn your next WP site into yet another WP blog.

It is very important to be able to customize your WP theme so that it can have a unique look and feel or at least purchase a premium WP theme that is not so widely used.

Genesis is terrible code to try and edit.
A better option would be to find a theme via ThemeForest.

Genesis is a Theme Framework. It gives you a lot of features for extending the functionality without writing much code.

Whereas Twenty Ten or any other free theme is built to be used as-is.

Genesis is meant for developers, who know their way around PHP, HTML, CSS and need to build custom solutions for their clients. Genesis helps cut down the development time because of the existing code base.

If I were to start writing a theme for a client, I could even modify Twenty Ten or any other GPL theme but the time I would spend building the same theme with Genesis will be much less than my custom theme.

Genesis has a bunch of ready-to-use child themes available on their website.

So, if you are not a web developer then you can pick any of their existing child theme + genesis framework. And use it on your site.

Another reason why developers use frameworks like Genesis is that the StudioPress team takes care of future development.

Let's say, I built a custom theme for a client, a year later some code that I used was deprecated in future version of WP. I need to go back to my client, update the theme accordingly and make sure that it works.

Now, if I had used Genesis, I need not worry about this. If something changes in WP core, StudioPress team will take care of their code in Genesis. They release an update to make sure that all users' site run smoothly.

Even my clients can update their site to latest version of Genesis. And my child theme built on top of Genesis will continue to work for them.

For more, I would recommend you going through What is the Genesis Framework? (And Other Genesis FAQs) | Bourn Creative

NOTE: Genesis comes with top notch quality of code, highly secure, extensible and optimized for search engines. You won't get that quality of code any where in a free theme or ThemeForest theme.

With genesis themes, you can modify your theme's color, features according to your need, but with free themes, you can't do this.