Tables are where the data is kept. For example there's probably a table of members. There are many rows on it, most likely one per member, and each row will have a specified number of columns which hold eg name, email address, date joined etc. Name might be used as a key to find your threads, posts you have made on other threads etc. It could be a hardware fault that caused it, of it could be that the connection is lost so you have eg posts without an owner. I worked on ICL, later Fujitsu's, database software and absolutely loved fixing database corruptions.