The active file lists all groups available on the server in question. It also lists the highest and lowest current article numbers in each group.
Here's an excerpt from a typical active file:
soc.motss 296030 293865 y
alt.binaries.pictures.fractals 3922 3913 n
comp.sources.unix 1605 1593 m
comp.binaries.ibm.pc 5097 5089 y
no.general 1000 900 y
Here's a pseudo-BNF definition of this file:
active = *group-line
group-line = group spc high-number spc low-number spc flag <NEWLINE>
group = <non-white-space string>
spc = " "
high-number = <non-negative integer>
low-number = <positive integer>
flag = "y" / "n" / "m" / "j" / "x" / "=" group
For a full description of this file, see the manual pages for ‘innd’, in particular ‘active(5)’.