KML is a file format used to display geographic data in an Earth browser, such as Google Earth, Google Maps, and Google Maps for mobile. KML uses a tag-based structure with nested elements and attributes and is based on the XML standard.|
KML stands for Keyhole Markup Language (Keyhole was the name of the application before Google bought it and added their own features and larger databases). If you understand HTML/XML you will have little problem understanding the syntax of KML. You can learn about KML from the KML Tutorial, or get the full details from the KML Documentation.
KMZ stands for KML-Zipped. It is the default format for KML because it is a compressed version of the file. One of the more powerful features of KMZ is that it allows any images you use - say custom icons, or images in your descriptions - to be zipped up within the KMZ file. That way you can share these details without having to reference the files through some link to the Internet. For KMZ files without images, the file size will be much smaller than the equivalent KML file.