How to Migrate a SharePoint 2007 Site to SharePoint 2010 Using Database Attach

Using the database attach method, we can migrate a site/site collection to SharePoint 2010 following some basic steps:

1.  Create a web application in SharePoint 2010 with blank site collection
2.  Backup the content database of SharePoint 2007 site.
3.  Copy the content database backup to the SharePoint 2010 SQL server.
4.  Mount that content database to the SharePoint 2010 server.
5.  Update the user interface.

Check the following source link for more detailed explanation with pictures:
How to Migrate a SharePoint 2007 Site to SharePoint 2010 Using Database Attach