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[jobeard]

We take a disk, partition it and then "format it" to make the HD usable to the system. But what does format really do and what does my choice imply?

Formatting the new partition creates a filesystem for the space allocated to the partition.
Great but what then is a filesystem?

• a couple of bits in the partition table written to the disk
  
• a low level rewrite of every sector in the space
  
• a module loaded at boot time (or mount time) that will support the features of that filesystem.

The filesystem is created using the mkfs -T ext{2,3,4} as noted below. The type is used to redirect to the filesystem specific configurator mkfs.ext2, mkfs.ext3, or mkfs.ext4 (you would likely guess correctly that mkfs.ntfs, mkfs,xfs and mkfs.vfat would also be valid).

Linux now has several filesystems available with different features:

ext2: supported by ext2fs filesystem driver

• 0x83 (Master Boot Record) EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  
• 2 ext2 data structures
  
  • 2.1 Inodes
    
  • 2.2 Directories
    
  • 2.3 Allocating Data
• 3 File system limits
  
• 4 Compression extension

ext3:

• 0x83 (Master Boot Record) EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  
• 1 Advantages
  
  • 1.1 Size limits
    
  • 1.2 Journaling levels (better reliability)
• 2 Disadvantages
  
  • 2.1 Functionality
    
  • 2.2 Defragmentation
    
  • 2.3 Recovery
    
  • 2.4 Compression
    
  • 2.5 Lack of snapshots support
    
  • 2.6 No checksumming in journal
• 3 ext4

ext4:

• 0x83 (Master Boot Record) EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  
• The ext4 filesystem is backward compatible with ext3 and ext2, making it possible to mount ext3 and ext2 filesystems as ext4. This will slightly improve performance, because certain new features of ext4 can also be used with ext3 and ext2, such as the new block allocation algorithm.
  
• 1 Features
  
• 2 Criticism
  
• 3 Delayed allocation and potential data loss
  
• 4 Compatibility with Windows and Macintosh

The full details of each filesystems are availble via the related links above to the associated WiKi articles.