Nomenclature
Formations are described by categorising the players (not including the goalkeeper) according to their positioning along (not across) the pitch, with the more defensive players given first. For example, 4–4–2 means four defenders, four midfielders, and two forwards.
Traditionally those within the same category (for example the 4 midfielders in 4–4–2) would generally play as a fairly flat line across the pitch, with those out wide often playing in a slightly more advanced position. In many modern formations this is not the case, which has led to some analysts splitting the categories in two separate bands, leading to four or even five numbered formations. A common example is 4–2–1–3, where the midfielders are split into two defensive and one offensive player; as such this formation can be considered a kind of 4–3–3.
The numbering system was not present until the 4–2–4 system was developed in the 1950s.
Read more about this topic: Formation (association Football)