Homestead improved uses Ubuntu as a base, but other boxes uses different distributions.
You can ssh into the server and run
cat /etc/*-release which will show:
DISTRIB_DESCRIPTION="Ubuntu 16.04.3 LTS"
VERSION="16.04.3 LTS (Xenial Xerus)"
PRETTY_NAME="Ubuntu 16.04.3 LTS"
For web development the distro often doesn't matter. Although once you start working on real projects, it's a good idea to create a VM that matches your real server, though distribution still matters less than installed PHP and MySQL versions.
There are occasions where distribution matters, for example if your server is running CentOS it has selinux enabled by default, Ubuntu does not. If your code is using unix sockets for anything, it'll work fine on ubuntu but you'll have to configure the centos server to allow it.