In Advance Wars, a Medium Tank is the second form of a Tank, with the third being Neotanks and the fourth being Megatanks. In Days of Ruin, the Midtank is followed by the Wartank while Neotanks and Wartanks are dropped.
Medium Tanks are fairly strong and great for holding certain points, including Bridges, Cities and a HQ, preventing opposing Infantry from capturing them until their defeat.
Offensively, Medium Tanks cause a healthy amount of damage to all but the Megatank. Several units can be severely damaged or destroyed in one hit when a Medium Tank is at full health, including Infantry, Recon and Indirect Combat Units, just to name a few.
Defensively, if a Medium Tank gets flanked by two or more Tanks, it'll take considerable damage. Indirect Combat is a great way for dealing with Medium Tanks if they can be drawn within range. But as good as their Defenses against Ground forces are, Bombers will cause massive damage to them.
Mid Tanks do very badly against Anti-Tanks in Days of Ruin.
Medium Tanks are great frontline units, but need the support of Recon and other units in order to see other opposing units in Fog of War because their vision range is limited to the space adjacent to them. Plus, their Fuel is less than that of a standard Tank so keep them near a Base or APC to keep them supplied.