2-Tailed vs. 1-Tailed Tests:
The purpose of a hypothesis test is to avoid being fooled by chance occurrences into thinking that the effect you are investigating (for example, a difference between treatment and control) is real. If you are investigating, say, the difference between an existing process and a (hopefully improved) new process, observed results that don´t show an improvement would not interest you so you do not need to protect yourself against being fooled by "negative" effects, no matter how extreme. A 1-tailed test would be appropriate. If, on the other hand, you are interested in discerning a difference between samples A and B (regardless of which direction the direction goes), a 2-tailed test would be appropriate.