Templates are pre-built presentations, professionally designed for specific topics.
When creating a new presentation choose "Use a template" to browse the library and select a template. You'll find different Category tabs on top, and more options for each template by hovering over them.
After choosing a template, you'll land on the presentation editor. On top, you'll find three settings to easily customize the look and feel of the deck: Theme, Palette, and Font. It's as easy as choosing the ones you like. Check these articles to learn more details about creating a custom Theme, and a custom Palette.
To change images, icons, and logos, just click on them to open the editing panel and click to choose a different image. To remove unwanted elements, just click on them and look for the Delete button, down in the editing panel; or just hit Delete on your keyboard.
The left sidebar allows you to move slides around, just drag and drop them. Click the arrow next to a slide's thumbnail to find options to delete, duplicate, or add new slides. You can also insert new slides and elements to the slides in the upper menu.