An inline tag does not cause a line break in the way that a block level tag does. Inline tags can be placed at the start, end or in the middle of the line. They may cause the line spacing to adjust to accommodate them if they are particularly high.
Here is a list of inline tags:
Some block level tags can appear to be inline tags if they control the alignment and text flow around them in such a way as to clear the alignment at one or other side.
Layers complicate things - they can make any block level tag appear to be an inline tag, however, positioning and text flow on separate layers is difficult to control effectively.