As you have probably read on this week's op-ed, or via various other channels of announcement, 3 April will see the introduction of the Typography refresh (or update) for the Vector skin on all Wikipedias. Other projects like Commons will have this update rolled out a few days before. You may also have tested the beta option. Basically, this means you will see some, and may see other changes on Wikipedia. In short:
The body font may show on a different font, depending on your operating system and installed fonts.
The font size and leading (line height) of body text are increased slightly.
The body font color becomes a very dark gray instead of black.
Pre-formatted text that is wider then the screen will show a scrollbar. [Removed last minute]
Other features that were in the beta but were not strictly typography-related, such as the restyled table of contents, thumbnail and blockquote styling, have been removed.
For most Windows readers, this means the headers will show in Georgia and the body text will stay in Arial, albeit slightly larger. For most Mac users this will be the same, but where the pages would normally show in Helvetica, it will now show in Helvetica Neue (it may be hard to see any difference).
On Linux, effects may vary. Where several distros and browsers all had their own default font settings, resulting in many different looks, the new typography should equalize them all to use Liberation Sans for body text. The headers may show in Linux Libertine, Georgia or Times, in that order of preference, depending on which of them are installed.
Georgia may look familiar; the Signpost has used it for its headers for some time now. And that is why...
Tech News highlights
You can now double-click or press the ↲ Enter key on an image or template to change its settings. 
Pages that don't exist will now show as red links in the edit mode. 
You will no longer be able to try to use VisualEditor on pages translated using the Translate extension, and you will see a warning if you try to edit pages marked for translation. 
You will now see a full warning, including the most recent log entry, when you try to edit protected pages with VisualEditor.