It's GPL3, so all rules of it apply.

<http://www.httrack.com/page/5/en/index.html>
<https://github.com/xroche/httrack/blob/master/license.txt>

Get original info on the license type here:
<https://www.gnu.org/licenses/gpl-3.0.en.html>

In short (this is no guaranteed legal advice!):
- yes, you can use it in corporate env, for free, no license fee
- but remember: if you apply any changes to the source code, you are obliged
to publish it back to this project


Dive into the legal details on the links given above.

Also check the details for a special situation: if you would not change the
HTTrack sources as such, but integrate/combine it with other software, you may
be also obliged to publish any software/sources, which you integrate it with.

Best would be if your corporate legal department gets familiar with GPL3 and
can advise you on it.