Bullet voting is a tactic in which the voter only selects one candidate, despite the option to indicate a preference for other candidates. They might do this either because it is easier than evaluating all the candidates, or (depending on the voting system in use) as a form of tactical voting.
If enough voters bullet vote, almost any voting system functions like a plurality voting system. This is generally considered a poor result, as many voting systems are intended as reforms or improvements that avoid or minimize some alleged disadvantages of plurality (aka first past the post). However, if voters bullet vote as a conscious strategy to express their meta-preference that candidates with strong support are preferable to "compromise" candidates with broad support, it may not be a problem.
This tactic is possible in any voting system that does not require ranking all candidates. In practice, this is the majority of voting systems as actually implemented (as opposed to in theory), since to reject ballots that do not rank all candidates risk an excessive number of spoiled ballots. However, it is mainly an issue in voting systems that reward this tactic—primarily approval voting, and to a lesser extent range voting—and in those that do not significantly penalize it—primarily instant-runoff voting and a non-standard form of Borda count.
Candidates may seek to encourage bullet voting in certain situations. One example is where there is a Bloc voting election for two seats of the same office, and there are several candidates (say A, B, and C). Voters in such a situation typically have two votes. Candidate A encourages his voters to vote only for him and not use their second vote. If the second vote is cast for B or C, it helps A's opponents. The situation is most pronounced where A is of one party and B and C are of another party. If voters from B and C's party vote for them, while A's partisans cast one vote for A and split their second vote between B and C, A is significantly disadvantaged.