What is the difference between `du` and `df` command?

df

• stands for Disk Free
• calculates file size based on the file system meta information
• if block size is 8kb and file size is only 1kb then file size for du is 8kb
• is faster but not accurate

du

• Disk Usage
• counts size used on the file system per file recursively (df and du giving different results)
• does not count open files
• does not rely on block size: 1kb file size with 8kb block size is still 1kb for du
• is slow, but more reliable

See this post from Mike Golvach for thorough explanation: Why DU And DF Display Different Values On Linux And Unix