I came across an article recently from MyFonts.com that helpfully explains the differences between ‘point’ size – which includes the ascenders (e.g., d, f) and descenders (e.g. q, p) – and x-height which is the standard height of the main letters (e.g. a, e, o, s).
The x-height variation among font types can greatly effect the readability of a specific font at certain sizes – which has important outcomes for site design. Download the PDF below to read more.