In the comments section of Does it matter if I use \textit or \it, \bfseries or \bf, etc, I asked for a clarification of these points. The clarification has mostly been made, but it was requested that I turn this into a question to provide for more room for this question to be fielded.
So here it is:
Please explain the difference between
\textit. When should they be used, respectively?
Those are very different commands even if the output happens to look the same.
If you want to emphasize a word or some text, use
\emph. Don't just make the text italic or bold. If needed, you may change the behaviour of
\emphwhenever you wish in the preamble and the whole document will be adjusted accordingly.
If you want to get italic text, use
\emphmight have a different effect, a package like
ulemmight change it to underlining for instance.
\emphmay be nested: emphasized text within emphasized text may be upright. In contrary, nesting
\textitjust keeps the italic shape.
Further, I rarely use physical font commands in my body text. I use them to define styles in the preamble and use the styles in the document afterwards, ensuring consistency and allowing changes to be easily made.