Approximately 20 years ago European VHF radio hams, meeting at Maidenhead in England, adopted a worldwide location-designation system proposed by a Swedish ham. The motivation was a short-hand for use in describing your station location, and to be used in contests: "Contact as many grids as possible."
It is a variable-precision system, with additional precision being obtained through appending further characters to the description. Characters come in pairs, the first character of the pair is longitude and the second is latitude. It is a "read-right-up" system; longitude to the east from 180 deg, and latitude north from the south pole.

The first pair of characters are letters, and identify "fields" of 20 degrees in long and 10 deg in lat, so there are 18 in each direction. I'm in JO.
The second pair of characters are numbers (0-9) and refine the fields into "squares" starting at the lower-left corner. Thus these are 2 deg in long and 1 deg in lat. In all cases, a "square" is identified by its lower left corner, there is no rounding. I'm in JO20. That's as far as USA hams usually go.
Most European hams continue to six characters. The 5th and 6th characters are letters and divide each "square" into 24ths, thus 5 by 2.5 minutes. I'm in JO20GU.

One can continue this process as long as you wish. Zero to nine for the numbers and A to X for the letters:

    [let][let][num][num][let][let][num][num][let][let]....

This quickly gets to more precision than is justifiable for most applications.

    Number of      Square         Pattern     Approx.     My Locator
    Charact.       Size                     Latitudinal
                                            Resolution
     2             20x10 deg         LL       1111 km       JO
     3              2x1  deg        LLNN       111 km       20
     6             5x2.5 min       LLNNLL      4.6 km       GU
     8             30x15 sec      LLNNLLNN      460 m
    10        1.25x0.625 sec     LLNNLLNNLL      19 m

(source : Bob Carpenter w3otc.nosp@m.@ams.nosp@m.at.or.nosp@m.g)