Transitions, transition words, or transitional expressions, et cetera, are certain words, expressions, or other devices that give text or speech greater cohesion by making it more explicit, or signaling, how ideas are meant by the writer or speaker to relate to one another.
Transitions can signal addition, example, contrast, comparison, concession, result, summary, time (often chronology), and place. The list transitions include the following: last, first, second, next, but, on the other hand, moreover, in addition, furthermore, however, to begin with, otherwise, conclusively, lastly, secondly, thirdly, most importantly, in conclusion, to end with, first of all, last of all, to sum it up, last but not least, lastly, finally, 'for example, and on top of all. (list incomplete)
Transitional paragraphs are devices used to bridge different topics discussed at length within the same essay or other piece of formal writing.
Handbook for Writers by Lynn Quitman Troyka and Douglas Hesse (Simon & Schuster)