De Bellis Multitudinis (DBM) is probably the most played set of rules for the hobby of ancient and medieval wargaming today, for the period 3000 BC to 1485 AD. DBM is at the more strategic end of the tabletop figure gaming genre, as its name implies (De Bellis Multitudinis = The Wars of the Many) it is aimed primarily at simulating large battles. The rules allow armies to be chosen from published Army Lists (in 4 books, with about 250 different army lists in total - but many more once all the in-list variants are taken into account) using a points system to select roughly equal armies if required.
DBM was written by the UK based Wargames Research Group (WRG) team of Phil Barker, Richard Bodley Scott and Sue Laflin Barker. It evolved from the simpler DBA ruleset in the early 1990's. The DBx series now also includes Hordes of the Things or HOTT (a fantasy version), and DBR (a Renaissance version).
DBM evolved from the earlier WRG 7th Edition Ancients ruleset, using its mechanism of fixed size elements as the basic fighting unit rather than individual figures, each element representing the smallest coherent fighting group possible. Any element in DBM covers the same frontage but at different formation densities.
DBM also broke with ancients ruleset tradition by defining troop types by function - defining troops as bladesmen rather than Roman legionaries for example - allowing a higher level of abstraction to be used across 5,000 years of warfare. In addition it defines Superior, Ordinary and Inferior troops not just by training and morale but also by battlefield effectiveness. These approaches, coupled with a fairly simple game system, produces a fast playing but subtle game that is easy to pick up but much harder to master, and mirrors ancient warfare a lot more closely than most rulesets that have gone before.
The armies are usually played in 15mm or 25mm scale, though 6mm and 54mm are used. Ground scale is in paces, and the number of inches to a pace varies according to the figure scale - 1" to 50 paces in 15mm, 40mm to 50 paces in 25mm. The frontage width of the element base is standardised for all troop types, the depth and number of troop models on it varies by formation type (light infantry - or psiloi in DBM terminology - have 2 men per base, cavalry 3, heavy infantry 4 etc)
Troop scale is not stated specifically, but as a the range of troops in an element ranges from 128 to 256, and the number of figures from 2 to 4, an assumed scale of c 1 : 60 is not way off the mark. Elephants, Chariots, Artillery and Shipping are 1 model per element, representing varying numbers of that type - for example 16 elephants or 25 chariots.
Although the game is designed to be used between historical enemies the level of abstraction, standardised army lists, and points system allows players to pit 5,000 years worth of opponents against each other with fairly believable outcomes in the main. This ability to match wildly disparate armies has always been a major part of the allure of ancient and medieval wargaming to some players, much to the horror of those gamers who believe that only historical battles (or at least much smaller spans of history) can be accurately simulated.
As well as friendly games, DBM competitions are played worldwide - including a truly global World championship. Competition games are typically played from 200 to 500 points, club games are typically 350 to 400 points. A 400 point army is typically an army of between about 50 and 80 elements, or about 150 - 250 figures in total (a high quality, mainly mounted army like a Mongol is more expensive per element, a mainly militia type such as Anglo Saxons at Hastings is cheaper per element and thus much larger).
The DBM community is global, a good starting point is author Richard Bodley Scott's webpage http://www.byzant.demon.co.uk/dbm.htm