This page is intended to explain the meaning behind each of VisIt's plugin types:
Note that expressions and queries are not yet implemented as plugins.
A database corresponds to a file format reader (or writer). This is how a user gets data into (or out of) VisIt.
Operators correspond to data manipulation.
Plots correspond to a rendering method. The tricky caveat with plots is that sometimes these rendering methods have an implied operation. The standard example of this is a contour plot. It renders contours, but must first extract contours to do this. (Contouring is its implied operation.) Note that this functionality is combined with a plot, since it directly affects how the resulting surfaces will be colored.
Why should there be a contour plot, but not a slice plot?
If you take the text above at face value, you could as "why isn't there a slice plot in VisIt?" The answer is that you never want to render a slice in any way dealing with properties of the slice. That is, when you contour, you want to color by levels. If you wanted to "color by slice", then you would need a slice plot. But, since you always want to color by a variable instead, no slice plot is needed.
Expressions refer to derived quantity generation. Anything that creates a new field is currently an expression.
Queries are the mechanism to put out quantitative data. This quantitative data is often in a reduced form (often a single number).