Although Powerline is extremely popular (and looks undeniably cool), it does take a bit of effort to install. It used to be distributed as a Vim plugin, but was superseded by a Python script that many struggled to install. The advantage of the Python script is it consolidates Powerline-style info bars for several applications: Vim, tmux, screen, and more. However, there were those that felt it was too laborious to get it set up right: messing around with patched fonts and Python is a barrier to entry.
That's where vim-airline comes in. It is 100% Vim script, small, integrates with other plugins like NERD Tree and fugitive.vim, and still looks good with unpatched fonts. I think for me that's a big win, because I often find myself using systems that can't even support the patched fonts Powerline works best with.
The author, Bailey Ling, maintains an airline wiki with useful hints.
And, if you're interested, my own hackish tmux Powerline-inspired settings look like this: tmux.conf. I used some colours and blank spaces to create what I thought was a nice effect, while looking fairly simple still.