A diverse and varied collection of philosophical theories, methodologies, and inquiries seeking to answer this family of questions "What is truth?", "What makes true beliefs true"?, "What is the difference between truth and falsity?", "How do we know true sentences are true?", etc. etc..
Most theories of truth attempt to provide answers to these questions and related questions, including: "What sort of things (or ontological entities) can be true (and false)?", "Is truth objective, subjective, or relativistic?", "Is truth knowable?", "Is truth divine, occult, or natural?", etc. etc..
There are almost as many theories of truth as there are schools of philosophy. The most popular theories of truth advanced by philosophers in the Western tradition are these, including some of the famous philosophers who were instrumental in their development:

Although I have included names like Friedrich Nietzsche, Michel Foucault, and Richard Rorty on the above topography, it can also be argued that their conceptions of truth do not approach the 'theories' proferred by the epistemologists and analytic philosophers populating the nearby geographies. In addition to the trend in philosophy that contemplates the nature or essence of truth, there is also the trend that argues that truth has no nature, it is varied, and mixed. Foucault, for example, tries to historicize truth and treat it as another concept that can be traded back and forth in exchanges of power. Rorty, on the other hand, tries to tie truth to the complex human practices and discourses in which it makes its appearance, thereby eliminating the redundant extrapractical theory that philosophy may produce.