There are different types of tactical voting:
Compromising (sometimes favorite-burying) is a type of tactical voting in which a voter insincerely ranks an alternative higher in the hope of getting it elected. For example, in a first-past-the-post election, a voter may vote for an option they perceive as having a greater chance of winning over an option they prefer.
Burying is a type of strategic voting in which a voter insincerely ranks an alternative lower in the hopes of defeating it. For example, in the Borda count, a voter may insincerely rank a strong alternative last in order to help their preferred alternative beat it.
Push-over or turkey-raising is a type of strategic voting in which a voter ranks a weak alternative higher, but not in the hopes of getting it elected. For example, in a bloc vote where multiple votes are required, a voter may insincerely vote for a candidate they perceive as unlikely to win, in order to help their preferred candidate win.
It has been shown that it is impossible for a voting method to be both strategy-free and deterministic. For example, the random ballot voting method, which selects the ballot of a random voter and uses this to determine the outcome, is strategy-free and non-deterministic.
Tactical voting is quite well known in United Kingdom elections. There are three main parties that are represented in the Parliament: the Labour party, the Conservative party and the Liberal Democrats.
Of these three, Labour and the Liberal Democrats are most similar. Many people who prefer the Liberal Democrats vote for the Labour candidate where Labour is stronger and vice-versa where the Liberal Democrats are stronger, in order to prevent the Conservative candidate from winning.
Most academic analysis of tactical voting is based on the rational voter model, derived from rational choice theory. In this model, voters are short-term instrumentally rational. That is, voters are only voting in order to make an impact on one election at a time, and they understand how best to use tactical voting to their advantage. The extent to which this model resembles real-life elections is the subject of considerable academic debate.
In many cases, it is difficult to distinguish between tactical voting by voters, and its analog before the election. For all the same reasons that voters might decide to vote tactically, campaign donors and activists may decide to tactically support or not support candidates with their money and labor, thus leading to results similar to those caused by tactical choices by the voters. A campaign can be sunk before it ever starts because it fails to convince enough sympathetic people that the campaign is viable, and hence worth backing.
Due to the especially deep impact of tactical voting in past the post">first past the post electoral systems, some argue that systems with three or more strong or persistent parties become in effect forms of disapproval voting, where the expression of disapproval, to keep an opponent out of office overwhelms the expression of approval, to approve a desirable candidate. Ralph Nader refers to this as the "least worst" choice, and argues that the similarity of parties and the candidates grows stronger due to the need to avoid this disapproval.
Some people view tactical voting as providing misleading information. In this view, a ballot paper is asking the question "which of these candidates is the best". This means that if one votes for a candidate who one does not believe is the best, then one is lying. Labour Party politician Anne Begg considers tactical voting dangerous:  (http://news.bbc.co.uk/1/hi/uk_politics/1091208.stm)
Steven Brams and Dudley R. Herschbach argued in a paper in Science magazine in 2000 that approval voting was the system least amenable to tactical perturbations. This may be related to the fact that approval voting does not permit preferences ('likes' or 'dislikes') to be stated at all, permitting only a statement of tolerances, that is, "which candidate could you stand to see win", as opposed to "which candidate would you like to see win".