Log truncation after Log backup

As we know, Once we take Transaction log backup, it will clear some space from the log file. So i have some confusion here, how it clears space once we take log backup.

I know few things, that Log file is composed of VLF, so once we take log backup Inactive VLF removed and that space is re-usable.

Is it right that once we take log backup, It contains only committed transaction or active VLF

I just want to know few terms below

1) What is Inactive VLF

2) What is active VLF

3) What is Committed transaction