in most cases the stain "is" the sealer.
Deck finishes fall into a few categories, and varieties. The two basic divisions are penetrating finishes and surface finishes. With a penetrating finish the wood provides the wear and tear and the finish provides color and weather resistance. A surface finish can be clear like a varnish or paint like.
Stains are typically a penetrating type. and can be transparent, semi-transparent or opaque.
The wood itself will vary in durability based on what type of wood it is. Old growth cedar and redwood will have more wear resistance than fast grown version or PT (pressure treated) as PT is often fast grown SYP ( southern yellow pine). My deck is a hardwood called Ipe that is about the most durable thing you can use for a deck. I treat mine with a penetrating oil finish called penofin...this maintains the brown color I like, other let Ipe weather to that dull grey with no finish applies at all.
@Jil D we have a cedar deck which has a grey washed stain on it, we also have a yellow lab and the two do not mix. The problem is the dog launches from the top deck and the nails scratch the deck. We have tried to protect it, even with outdoor carpets, to no avail. We have settled on staining the upper deck every spring and it lasts until sometime in the winter, but still have to redo it in the spring. We clip his nails, but perhaps not enough - cut too short once and it started to bleed for a short time, maybe we should clip more often ???
Thanks Gail, Nice to know someone else is in the same boat with the toe nail scratches. Maybe yearly is the best its going to get.