ESPN doesn't really make it easy to understand, especially with renaming their packages differently over the years (i.e. ESPN 3 => ESPN + => ESPN Select). The easy thing to understand is that ESPN Unlimited gives you access to all of their content. ESPN Select is where it gets less clear. According to ESPN, this is how they differentiate ESPN Select from Unlimited...
Select does not give you access to the full set of linear ESPN channels (like the flagship ESPN channel, ESPN2, ESPNU etc) as a live stream
Maybe it's just me but that's not exactly clear.