qa_userevents table contain all event information aobut the user such as question, answer, comment etc..
Edit:
Refering to the table
entity type will store the entity type
Question as Q, User as U, Tag as T, Category as C, If none as -
While updatetype column will store some kind of event information for instance update category for the question, question closed, content edited etc.
You can find more details in qa-app-updates.php file