Moisture retaining is anything that holds water, organic or not, but usually ends up being organic, clay (or other fine mineral particulate) is an example of a non-organic one. Some organic examples were given above.
Aerating is anything that is a chunk, pebble, or particle of a certain size to provide a pocket of air around it. This is perlite, pumice, coarse sand, or other rock material usually, rice hulls were mentioned above for an organic one.
Some provide both aeration and some water retention, such as vermiculite, wood chunks, or clay pellets. These usually don’t hold enough water to fall under the moisture retaining category and go under aeration. But it is good to know that they hold some moisture just in case it matters to the grower. Usually things that can both hold water and provide aeration compress over time.
So you add up the percentage of each type of material included to calculate the ratio, they should add up to 100 together. Usually this applies to whatever is providing bulk to your mix and not any small amounts of amendments.
Example:
Aerating materials:
Perlite 20%
Pumice 20%
Sand 10%
= 50% aeration
Moisture retaining materials:
Peat 30%
Coco coir 18%
Compost 2%
= 50% Moisture retention
50 + 50 = 100