Since you used an empty third argument, no label is used; in particular, no string "Chapter" (or "Appendix") will be used for the chapter headings and no counter will appear; notice, however, that the counter won't print in the title but "it is still there" (it will be show in The ToC, for example).
No optional first argument was also used in your definition, so the <shape> will default to hang, which is the shape used by section headings in standard classes.
In your description you said that you only wanted to suppress the string "Chapter" (or "Appendix") but keeping the numbering, so a better option would be to use something like
\titleformat{\chapter}
{\bfseries\LARGE}
{\thechapter}
{0.5em}
{}
Notice that I added \thechapter in the third mandatory argument, so the counter will be typeset and also I moved \bfseries\LARGE from the fifth mandatory argument to the second one; in this way the formatting will affect both the label (the counter, in this case) and the title; with your settings (used in the fifth argument) it would only affect the title but not the label.
A complete example:
\documentclass{book}
\usepackage{titlesec}
\titleformat{\chapter}
{\bfseries\LARGE}
{\thechapter}
{0.5em}
{}
\begin{document}
\chapter{Test chapter}
Some test text
\end{document}
The result:
As a side note, \bf is a TeX command which shouldn't be used in LaTeX documents; use \bfseries instead in LaTeX2e documents.
The documentation of titlesec explains, in page 4, the general syntax and the meaning for the arguments of \titleformat. I see no point in repeating here what appears there.
titlesecdocumentation is not clearly laid out, unfortunately. But here the answer is pretty simple: The first set of empty braces effectly tellstitlesecto have no "format" (in the sense meant by the package); the next empty braces do the same for "label". Also, since you are using LaTeX, use\bfseries, not\bf. – jon Mar 07 '15 at 21:23