To save your work, press F2. The current window changes into a FileSelect. The top two buttons are needed to enter a directory and a filename. You can leave these buttons with an 'enter'. Press 'enter' again to save the file. If the file exists, a requester pops up to confirm a save over. 
A faster way to save without FileSelect is CTRL-w. 

To load a blender file: press F1, move the cursor to a file and press MiddleMouse or click LeftMouse and press 'enter'. A faster way to load without FileSelect is CTRL-o.

There are two ways Blender protects the user for software failures and save errors:
1- file versions: in the user presets menu (top header, pull down edge) there's a versions button. Every time a file has been save, first the old file(s) will be renamed.
For example: the file rt.blend becomes rt.blend1. 
2- auto save: with this option, also in the user presets menu, every 'x' minutes a temporary file will be written. The filename has a unique number (the process id). After successfully quiting Blender (press 'q') this file will be renamed as 'quit.blend'. 

Blender has a - sometimes frustrating - preservative memory management. 
Deleting an Object for example, doesn't mean everything that has been linked (Meshes, Materials...) directly is freed in memory. After a few hours working it's possible to have a lot of zero-user data blocks. 
This can be used effectively to 'undo' a deletion: use the browse button in the headers to restore a link.
Data blocks that have zero users are NOT written in files. 

Only when a file is loaded, memory will be freed and reorganized. 
 

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