Japan was introduced in World of Tanks update 0.8.10, and until the introduction of the general European tree will remain the newest tank nation in WoT. Currently, there is only one end-tier medium line.
There are a few things all Japanese tanks have in common. They all have poor armor thicknesses, but the presence of sloped and rounded armor zones can lead to lucky bounces. They also combine good gun depression with guns that have excellent penetration and a good balance between alpha and ROF. In the first few tiers, the Japanese tanks can seem to prove to be a real turn off. They have somewhat poor aim times (though certainly not as poor as Russian tanks) and accuracy that can leave a good amount to be desired. However, most Japanese tanks have decent camo values for their class.
At tier II, the line splits between medium tanks and light tanks. Japanese light tanks offer good speed and agility, culminating into the Type 5 Ke-Ho, which is small, fast, agile, and packs a surprising punch. However, to balance these overall good characteristics, the three light tanks in the Japanese line lack in armor even more so than their medium counterparts, and are not exactly specialized for scouting in general.
In the diverging medium tank line, agility is traded for additional firepower and flexibility, a shining example being the Type 97 Chi-Ha, which while albeit cumbersome, possesses great firepower and accuracy for its tier, which enables it to play not only as a support medium but also a tank destroyer. The Medium tanks, along with their light tank cousins, are somewhat lacking in armor overall compared to their international peers.
At tier V, the medium and light tanks lines reunite, and the Japanese tank line focuses solely on mediums which have decent accuracy as the line reaches later tanks, all of which have the capability to switch roles seamlessly when required. Past tier V, the medium tanks take rather wild variations between tiers. The tier 6 Chi-to is known for its overall good maneuverability for its size. The tier 7 Chi-Ri is notable for its very large size, rivaling the bulkiness of the German tanks found in the E50M line, being even bigger than some heavies of its tier, which makes it an enticing artillery target. Yet to trade for its large size, the Chi-Ri has the ability to equip dangerous autoloaders, the earliest medium tank in the game to do so. In addition, to compensate for these wild variations, all of the late-tier Japanese tanks feature above-average to good health pools, some rivaling those found along the E50M line as well.
In the last few tiers, the turrets of the Japanese tanks, while not greatly increasing in armor thickness, begin to become well-rounded and sloped like their Chinese counterparts, the STB-1 for instance has among the shallowest sloped turrets in the game, able to bounce nearly any shot. They also likewise have cramped modules and crew. However, in contrast they still retain their excellent gun depression but also abysmal hull armor. In essence they are not jack-of-all trades and masters of none like the Russian Mediums, but are extremely potent as second line support tanks, and if handled properly can even take on primary roles in a pinch, if only temporarily.
Overall those who expect to see Japanese tanks brawl effectively will be generally disappointed and surprised, as all of the tanks in the Japanese line lack strong armor found in Russian and German tanks that will be sufficient for such. However, those who prefer a supporting role, sniping, flanking, etc. will very much enjoy what the Japanese have to offer.