Projects that use a version control system can have two types of log for changes. One is the log maintained by the version control system: each time you commit a change, you fill out a log entry for the change (see Log Buffer). This is called the version control log.
The other kind of log is the file ChangeLog (see Change Log). It provides a chronological record of all changes to a large portion of a program—typically one directory and its subdirectories. A small program would use one ChangeLog file; a large program may have a ChangeLog file in each major directory. See Change Log. Programmers have used change logs since long before version control systems.
Changeset-based version systems typically maintain a changeset-based modification log for the entire system, which makes change log files somewhat redundant. One advantage that they retain is that it is sometimes useful to be able to view the transaction history of a single directory separately from those of other directories.
A project maintained with version control can use just the version control log, or it can use both kinds of logs. It can handle some files one way and some files the other way. Each project has its policy, which you should follow.
When the policy is to use both, you typically want to write an entry for each change just once, then put it into both logs. You can write the entry in ChangeLog, then copy it to the log buffer with C-c C-a when committing the change (see Log Buffer). Or you can write the entry in the log buffer while committing the change, and later use the C-x v a command to copy it to ChangeLog (see Change Logs and VC).