Vim has netrw as a built-in directory browser. It is pretty basic and some people like that. Others find it a bit too basic or unintuitive. Tim Pope wrote vinegar to "enhance netrw, partially in an attempt to mitigate the need for more disruptive 'project drawer' style plugins". 1
A very popular tree explorer plugin is NERDTree. It is very pretty and has a lot of features but I find it a little too heavy for my taste.
I prefer to use FileBeagle as my directory browser. It is simple and efficient. As stated by its developer:
Vim is a text editor, not an operating system that can edit text. FileBeagle respects this, and attempts to conform to this both in spirit and in practice. If you are looking for a plugin to serve as a filesystem manager from within Vim, FileBeagle is not it. FileBeagle does not support copying, deleting, moving/renaming, or any other filesystem operations. FileBeagle lists and opens files. 2
The - key is mapped to open FileBeagle on the directory of the current buffer. You can then use the standard Vim navigation keys to move to a file or a directory or press - to go to the parent directory. Pressing Enter will either go into the selected directory or open the selected file for editing. There are many more functions available. I refer you to the plugin's documentation for more info.
I like FileBeagle to display hidden files and relative line numbers. Here are the variables I set in my .vimrc file to accomplish that.
let filebeagle_show_hidden=1
let filebeagle_show_line_numbers=1
let filebeagle_show_line_relativenumbers=0
Give FileBeagle a try. You might like it.