What are chess tournament categories?

If you are a chess fan, you probably already know what the tournament categories mean. MTel masters, which is going to start this month, for example, is a category 20 tournament (based on January rating list). The tournament categories are determined by the average tournament rating. Lets say, the average ELO rating of a tournament (sum of all ELO ratings / number of players ) comes to 2253, it fits the definition of a FIDE category-1 tournament.

Each category corresponds to a 25-ELO range. The ranges that are normally used by FIDE are:

Category	ELO Range
Category 20	2726 - 2750
Category 19	2701 - 2725
Category 18	2676 - 2700
Category 17	2651 - 2675
Category 16	2626 - 2650
Category 15	2601 - 2625
Category 14	2576 - 2600
Category 13	2551 - 2575
Cat 12	2526 - 2550
Cat 11	2501 - 2525
Cat 10	2476 - 2500
Cat 9	2451 - 2475
Cat 8	2426 - 2450
Cat 7	2401 - 2425
Cat 6	2376 - 2400
Cat 5	2351 - 2375
Cat 4	2326 - 2350
Cat 3	2301 - 2325
Cat 2	2276 - 2300
Cat 1	2251 - 2275

The table can be further extended to Category 21, 22 et al. The tournament categories were once used for GM and IM norms, but are no longer used (Check this page for the current FIDE title norms). As far as I know, the highest category FIDE tournament so far has been Category 21. If you know of anything more, do let me know.

The USCF (US Chess Federation) and ACF (Australian Chess Federation) maintain a separate scale for national titles and rating classification. You can find more info on these here