Who's Online Timeframe (general) is how long since they've been on; as the page data is cached it basically gives you the option of how long to list users being online; some sites like to show how many users visited in the day, in the hour, etc.
The cache time (in portal settings) is for how long the data is on the island is cached; this would be the basic stats that show on the island itself (how many users who where online and all that fun stuff)
