For most item details pages, there is a "Show History" link in the upper right corner. Clicking the link opens a vertical pane on the right side of the screen showing all the history for that specific item.
This history contains the same information available in the Account Activity reports in the Reports module, but filtered for this specific item. You can search the history by entering text in the "Search History" box, or you can select specific dates to view from the "Add Dates" menu.
When viewing a matter's history, you can select the "Show Full History" link, which will open the Reports module showing all activity related to that matter, including updates to tasks associated with that matter.