Wet or dry? We're not talking weather, but whether you will be using one or the other of these seed saving techniques.
As a rule the "Wet Technique" is used for seeds that "live" inside fleshy fruits and need to be freed from the gelantinous casing that prevents them from germinating too soon. Think about a halved tomato and what the seeds look like...that coating is nature's genius at work. Let's suppose a tomato falls from the plant in its ripest state in August. If the seeds from that fruit (yes, it's official, a Tomato is a fruit...the reason can be found here) fall to the ground and begin to germinate, there's not a snowball's chance in Hades that it will grow to maturity (and make mature tomatoes with their resident seeds) before cold weather sets in and is able to get its offspring out into the world. In nature, this work is done by the slow wearing down of the casing by weather or, if eaten, by the stomach acids of the tomato eater (birds or other critters).
Wet seeds can also be found in other vegetables, such as cucumber, eggplant, and squash. We humans who want to save seeds from these plants must find an alternative to wearing down this compounds, and so we use the Wet Technique.
The "Dry Technique" is just as it sounds. Leaving the seeds on the plant until they are (you guessed it) completely dry. Collect pods and seed heads on a dry day and put them in a paper bag to get them really dry. The Dry technique includes an additional step of winnowing, which aids in separating the seed from the husk or other plant material that can reduce shelf life and invite pests. Here is a nice overview of the Dry Technique.
Dry seeds include beans, peas, peppers, herbs and flowers. Beans and peas must stay on the plant until the pod becomes brittle and brown and the seeds inside are hard and dry. Peppers must reach full ripeness (i.e. not green).
A good overview of the steps for both the wet and dry seedsaving techniques is here.
A very nice summary of the importance of plant selection for saving seeds is here.
Along with requisite recordkeeping of what you grow in the garden, it's just as important to keep track of what seeds you've saved from it!
Keeping track of your seed stash can be as simple entering the information in a notebook or, for the more tech saavy, a spreadsheet.
Be sure to include the following information:
Remember to transfer seeds to an envelope with the same information after they have thoroughly dried.
Store your seeds in a cool, dark place and start dreaming about next year!
Depending on the type, seeds can have a life span of one year (onions) to more than four years (tomatoes) and how you store them can significantly alter how long they will stay viable (able to germinate and grow with vigor to maturity).
Generally, storing seeds in a cool/dark/dry place will help to keep seeds viable for their respective maximum life-spans. Seedsavers Exchange provides sound advice on storing seeds. Another nice overview of best practices in keeping seeds in tip-top shape can be found at Rodale's Organic Life magazine.
A handy seed viability chart for vegetables, flowers, and, fruit can be found at Fedco Seeds, High Mowing Seeds, and Clearcreek Seeds. Note: there is variability in reported viability. By being consistent with how your store seeds and making notes you will be able to more accurately assess the viability of the seeds you keep in your particular seed saving environment.