Dear Carmen, I think most of use here are wondering why you're asking such a question. A simple google search will give you many useful links as to what it is. If you have a question about a specific aspect of HTML5, sure, fire away - but what is HTML5 is like asking what's an elephant. Sorry if you find this patronizing, it's not meant to be.
I'll be brief with my own half-cocked explo:
HTML5 decades, it seems, in the making, comes of the heels of HMTL4.x and XHTML. It's a markup language subset of SGML. To all intents and purposes, you wouldn't really tell the difference between HTML4 and HTML5 from looking at a basic page other than the DocType Declaration. However, HTML5 comes with a plethora of additional attributes/properties and a few new tags. It also lends itself to working with js moreso that the other doctypes. There's hell of a lot more to it, but those are the bare bones as I see them.
Some attribute/properties are not supported by all browsers and older browsers may not support any of them. This shows a compatibility table for html5: http://caniuse.com/#cats=HTML5