What's the directory structure on disk that keeps the information of all files?
Your intuition is mostly correct. I believe the description is not really sufficient. Directory entry should at least be able to allow the OS to be able to locate some parts the file on disk for I/O operations. File System is divided into some fixed size partitions of available space ranging from 1-16KB. For efficient implementations a file is not necessarily written in contiguous blocks therefore the challenge is mostly having an efficient way of determining which blocks belong to which file without giving up on too much space and being efficient at the same time. Similarly other meta information should also be kept such as user permissions, hard links and so forth.

Currently Unix inode system is an efficient implementation. An other example would be maintaining a FAT (File Allocation Table) which is less efficient. I believe reading about Inodes would be great place to start.