MekWars is a campaign and matchmaking system "extending" MegaMek to include more strategic and long-term aspects beside the MegaMek tactical ones. This is done by providing a hangar for each player, holding units that may need repairs or reloading ammunition after a fight as well as other aspects like a map with locations to fight, the option to set up factories producing new units and purchasing them via a virtual currency afterwards.
MekWars can also automatically set up connections and hold and provide a list of dedicated hosts, those are MegaMek servers which host the actual tactical game. This setup eliminates most of the troubles many players experience when trying to use MegaMek over internet connections without properly configuring their firewalls or routers first.
After matchmaking in MekWars (which has a chat, lobby, factions, etc.) the MekWars server passes the data (units, settings) to MegaMek.
The entire system is written in Java, and for that reason is highly platform independent at the cost of rather high resource usage.
Several different Megamek servers used to exist. For various reasons, all but the Mekwars 3025 are no longer active. It is unknown if the others will return. There are some efforts to stand up a First Succession War server. It is not publicly available at this point.