Rock climbs are graded from easy to extremely difficult. Several different grading systems are used depending on the style of climbing. Sub-grades are often used such as 7a+ which is harder that a 7a but easier than a 7b:
Sport Climbing: The French grading system is widely used for sport climbing ranging from F6a, F6b, F6c which are easy routes, to F7a, Fb, F7c which are middle grade, through to F8a, F8b, F8c which are very difficult. The hardest routes are F9a and F9b.
Traditional Climbing: The English Trad. System is used here in the UK which has two components; the adjective grade and a technical grade.
The Adjective Grade is an overall grade to reflect the overall difficult and seriousness. The easiest grades are Moderate (M), Easy (E), Difficult (D) and Very Difficult (VD). The middle grades are Severe (Sev), Very Severe (VS) and Hard Very Severe (HVS). The hardest grades are Extremely Severe, which are split into E1, the easiest, through to E11, currently the hardest.
The Technical Grade is a measure of the hardest moves on the route. The easiest grades are 4a, 4b, 4c, the mid grades are 5a, 5b, 5c, the harder grades are 6a, 6b, 6c and the hardest of all is the seventh grade, of which 7a and 7b are the hardest which exist at the moment.
Bouldering: The Fontainebleau Grading system is widely used for bouldering. The easiest grades are Font 3, 4, 5, whilst the middle grades are Font 6a, 6b, 6c and the harder grades are Font 7a, 7b, 7c. The hardest grades at the moment are Font 8a, 8b, 8c.