town charters
note, too, that charter has a very broad range of meanings. In a basic sense it referred to any document granting, conveying, or describing rights of any sort, whether in land, income, tax privileges, etc.
When you ask about a 'town charter' you are referring to a specific kind of document in which a ruler (usually) grants specific privileges to a pre-existing urban agglomeration. The ruler would define in the charter the extent of those privileges. Usually the townsmen sought to gain a limited degree of self-government and internal judicial authority; in return for generous payments, the ruler might grant such privileges.
Such documents are extremely common for western europe beginning circa 1100. For one such charter, along with other documents describing how ephemeral such privileges could be, see:
http://www.uncg.edu/~rebarton/beauvaisdossier.htm
Charters to found new towns (as in a wild-west, or wilderlands sense) are pretty rare, although it is not uncommon to find a political ruler granting a charter of sorts to a 'burg' which has grown up around a castle or in a suburb.
It is also clear that rulers could rescind town charters, which meant that the town would henceforth be under the direct rule of that political figure. Here's a nice example:
http://www.uncg.edu/~rebarton/etampescommune.htm
Geleg