We have some great waterproof and weather-proof items in the adafruit shop, but once you have a project built, you'll want to enclose it. But how to keep an enclosure weather/waterproof while still attaching cables for power, data or sensors? That's where cable glands come in. By installing these over the cable, they form-fit to make a seal, keeping your project box innards safe from the outdoors.
We have two sizes, the PG-9 size is good for 0.158" - 0.252" diameter and this one, the PG-7 size, is good for cables 0.118" - 0.169" diameter. These two are the most common sizes for power & data cables, be sure to measure them so you have the proper fit.
Adafruit Best-Friend-Forever (BFF) Akiba over at Freaklabs.org has a great tutorial about how to use cable glands to keep waterproof enclosures waterproof, even with power or data or sensor cables coming out. Please check it out for some excellent information about how to use them!