Let me put it this way, Eddie...
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Tanks are heavily armed and armored full tracked vehicles intended for front-line combat. There are other tracked units (such as the Flak Track, Bullfrog or Specter) but these are not regarded as tanks because they are either half tracks or not intended for front-line combat.
There are exceptions: there are tanks designed for a supporting role and/or ones without full tracks but are regarded (or explicitly labelled in-game) as tanks nonetheless. E.g. The Robot Tank has no tracks but is regarded as one because it fills a similar role. The Spider Tank is explicitly called a tank in-game.
Generally, they are heavily-armed and heavily armoured compared to other vehicles but slower and designed for front-line combat rather than support. Contrast with Mechanized walkers.
Main Battle Tanks
For some strange reason, none of the tanks in Command and Conquer have a machine gun, probably for balancing purposes.
Refer to the following articles for more details: