If you are used to open projects in Project Professional in 2010 by double clicking on a project in the open window, you should know that the project is by default opened in edit mode and though checked-out.
Project Server 2013 is bringing here a nice improvement that I, as administrator, am really happy to welcome.
All Projects in Project Professional are by default opened in read-only mode and can be opened in edit whenever you want.
Then, while opened in Read Only , as for Office in 2010 and 2013, Microsoft introduced the "Notification bar" in Project 2013 allowing you to check-out the file.
I already hear you saying: "What about my modifications made while in read-only?". Don't worry, Project is clever enough to just check-out the project without reloading it. Any modification made in read-only mode are kept while checking out the project file
What if a co-worker changed the project in the meantime? Project will ask you to save you changes.
if you choose yes, it will ask you to find a place to save it before checking-out the file
You will have to pay attention while using the "Recent Projects" opening, by default, it will check-out the file for you!
From PWA the options are still the same : while clicking on the little workplan icon, the project will open in Edit mode to your computer MS Project and clicking the project link will open in Read only in PWA.
You will never again lock a file without noticing it!