Currently this term is considered to derrogatory by many Conservatives as many UK voters associate it with uncomplementary recollections of the governments of Margaret Thatcher and John Major. This is quite ironic given the origins of the term "Conservative" (see below).
Tory is unlikely to fall from common usage as newspapers find it too useful as an alternative for Conservative when space is limited.
The term originates from the Exclusion Bill crisis of 1678-81 - the Whigs (initially a insult - whiggamor (cattle driver)) were those who supported the exclusion of James II from the throne and the Tories were those who opposed it.
By the late eighteenth century the Tories were defined as believing in the divine right of Kings to rule. Whigs believed that the King was there by the goodwill of the powerful families and so could only continue at their approval.
Generally, Tories were associated with landed wealth, Jacobite sympathies, the Church of England. Whigs were more associated with trade, money, expansion and tolerance. Both were still committed to the political system in place at that time. Neither group could be considered a true political party.
The Tories had the name "Conservative" applied to them as an insult and later on they adopted this name as the official name of their party.