With SharePoint 2013 Microsoft has decided to remove the option for in-place upgrades from the previous version. This means that upgrading from previous versions of SharePoint will require building a new SharePoint 2013 environment and migrating all of the databases from SharePoint 2010 by attaching them to the new 2013 farm.
There is a step-by-step guide on migrating databases from a 2010 environment to 2013 here: http://technet.microsoft.com/en-us/library/cc263299(v=office.15).
A couple of notes:
Only SharePoint 2010 databases may be attached to SharePoint 2013. SharePoint 2007 databases will need to be upgraded to 2010 first and then upgraded to 2013.
The databases from 2010 Service Applications can be migrated to 2013 (services such as the User Profile Service Application and Managed Metadata Service Application).
Custom solutions for SharePoint 2010 would need to be deployed to 2013 separate from attaching the databases. The solutions will not automatically move with the databases.
Sites upgraded from 2010 to 2013 can run in full 2010 compatibility mode (unlike 2007 sites running on 2010 that simply kept the visual elements). This will allow solutions from 2010 to run seamlessly.