The last category, History, is fairly self-explanatory. History rows are what current and future rows eventually become.
If John's marriage didn't work out and he got divorced, a row would be inserted to his status field indicating a divorce, with an effective date defined by John. The marriage date, which was the current row, would then become the history row.
History rows enable you to maintain an accurate online history of your data. This data can then be incorporated into reports or viewed online.