Alpine climbs are generally classified as grade I to VI, with grade I being a climb of any technical difficulty that only takes a few hours and grade VI being a multi-day excursion with difficult free climbing. These grades were developed in the United States by the National Climbing Association System (NCAS) and take into consideration the length of a climb, the number of pitches, average pitch difficulty, difficulty of the hardest pitch, ascent time and route finding problems.
Ice Climbing Grades
The rating system for most water ice climbs is listed in Roman numerals but does not relate to the Roman numerals of the alpine rating system. Guidebooks will often list ice climbing ratings with the letter "W" before the Roman numeral to signify what kind of rating it is. Numbers I though VII are used, with I being an easily accessible climb near a road that is short and easy with no avalanche hazards and VII being a climb that may take days to reach and is both mentally and physically a great challenge. A VII climb has objective hazards present at all times.
Tecnical Ice Ratings
A technical ice rating is one that is defined by the most difficult pitch. These are numbered 1 through 8. As defined by "Mountaineering: Freedom of the Hills" (7th edition), a technical rating of 1 would be a perfectly flat area of ice such as a frozen lake or ice rink, a 4 is a sustained pitch that is nearly vertical or vertical and may have features such as chandeliers or run-outs between protection and an 8 is a thin, gymnastic, overhanging route that would only be completed creatively.
In New England, water ice has its own rating system: NEI I to NEI 5+. NEI I is classified as low-angle climb (40 to 50 degrees) or a long moderate snow climb which can be completed with basic technical level. NEI 5 and up includes serious multipitch routes with long vertical sections of extreme difficulty.
Other Rating Systems
There are other rating systems that are unique to specific areas such as the Alaska ice rating system developed for the area's severity and own weather systems (among other things) and the IFAS (International French Adjectival System) that originated in the Alps and has six categories from F (facile or easy) to ED (Extremement difficile or extremely difficult). There is also a mixed climbing rating system developed by Jeff Lowe which is from M1 to M11+.