Available, i.e., the total storage of your disk.
Drive capacity: the amount available for store and write, excluded other space, used for software at low-level (i.e. Recovery Disk), which you can access to restarting with Command-R).
Free space: the space available to write into your disk and store your files before it becomes full.
Maybe I should be a bit more specific:
Currently the Get Info for Macintosh HD shows the following:
Capacity: 249.82 GB
Available: 282.69 GB
Used: -- bytes (-- KB on disk)
While for example df -k shows the following (which is accurate):
$ df -k
Filesystem 1024-blocks Used Available Capacity Mounted on
/dev/disk1 243966468 152025216 91685252 63% /
devfs 181 181 0 100% /dev
map -hosts 0 0 0 100% /net
map auto_home 0 0 0 100% /home
localhost:/ezioMnNoyRVU8Vj-YxC4cJ 243966468 243966468 0 100% /Volumes/MobileBackups
No matter which way you look at it, the Finder currently calculates the available (free) space incorrectly.
This is probably like the issue that I was having on my MacBook Pro. If you are using Time Machine on a laptop it stores the diffs in a file on your hard drive. This action can be disabled with the terminal command
sudo tmutil disablelocal
This turns the local Time Machine off. It is up to you if you want to do this, but Finder shows this space as unused since it can be overwritten when needed. It saved me over a hundred GBs of space.