There is no easy definition of citizen as requirements varied from place to place and over time (see below). A burgess was "defined as those who “practise mechanical and handy crafts,” cannot be considered gentle because they must serve an apprenticeship to learn their craft, and “during the time wherein they are bound to service for the apprehending of that craft, they seem as bound and of servile estate…"
A city would have been a populated area with a charter of incorporation, a definition of borough:
Derives from the Old English term burh, which was originally applied to any fortified place, such as a thegn's house or a hilltop – not necessarily a populated place. During the wars between Anglo-Saxons and Danes it was particularly applied to centres of population which were protected with defensive earthworks; such of those centres which continued to flourish in later times came to be boroughs. The term burgus often appears to be used in a technical sense, as distinct from "town" (villata), such as in the 1200 proceedings at Ipswich, usually relating to the town as a legal/administrative entity. Another term sometimes used in a general sense, but sometimes with a more technical meaning, was liber burgus, "free borough", (or the slightly older liber burgagium, referring to burgage tenure); this came into vogue at the close of the twelfth century and was popular throughout the thirteenth, when many communities sought this status through charters from their lords. It involved the grant of special privileges reducing the degree of servitude of burgesses to their lord and/or increasing the degree of corporate autonomy from external authorities; the precise set of privileges varied from town to town. This status was most commonly given to new town foundations, and implied their acquisition of privileges already in possession of many of the older, established towns (a specific town might be used as a model for a new foundation). Some historians have preferred to restrict “borough” to towns whose urban status had legal recognition, notably through their endowment with charters granting typically urban privileges; they use terms such as "market town" or “non-burghal town” to identify settlements that lacked charters but had other characteristics considered urban. http://users.trytel.com/~tristan/towns/glossary.html; burgess
Although Domesday seems to have used this term fairly consistently to apply to town residents contributory towards the customary payments due the king from boroughs, later in the Middle Ages its varied application does not suggest a precise, universally agreed, technical definition. Broadly, however, it referred to residents of a borough, usually those residents who were members of the borough community in terms of sharing in communal responsibilities and rights; hence we often find the term "comburgess" used, to emphasise that an individual was a fellow member of the enfranchised community (although the term also came to be used, on occasion, to refer to burgesses of higher status). At Lynn the poorest townsmen were clearly described as non-burgesses, "burgesses" evidently being equated with those residents who had become freemen; this appears also the case in Ipswich. Yet in Colchester the same class of poorer residents was described as being burgesses. Outsiders ("strangers" or "foreigners") were sometimes allowed to acquire some of the same – notably commercial – privileges by entering the franchise under the special status of "foreign burgess". Towards the end of the Middle Ages "burgess" was more likely to be used to distinguish one group of privileged townsmen from a less privileged group.; Some historians have connected the status of being at scot and lot with householding; this, however, was not a prerequisite (although in some towns it later became a requirement of citizenship), but there was a natural concern that members of the community have property – whether real or moveable – by which they could be distrained if they failed to pay their contributions...It is evident that "scot and lot" is not, as in Brewer's definition, two terms with essentially the same meaning, but presents two separate yet related concepts. The two may represent dual responsibilities, of contributing to taxation and bearing one's fair burden in administrative duties (the latter, as a pivotal element of the definition of citizenship, traceable back at least to the time of Aristotle, whose Athens of course provides the best-known government in which officials were chosen by lot), or the opposites of obligation and benefit; perhaps "lot" itself communicated more than one idea.

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Note added at 32 mins (2011-11-01 09:13:38 GMT)
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nota bene: The expression "to get off scot free" seems to come from "scot and lot."

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Note added at 53 mins (2011-11-01 09:34:27 GMT)
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BD has provided a good reference. As all these sources note, these categories were flexible and changed from place to place and over time as certain groups were given privileges, responsibilities, and exemptions. A family of craftsmen could become a family of merchants, for example.