Before you buy, ask yourself these questions.
What's the wood coated with now?
If it's bare, chose whichever stain you want. But if it has a film finish, it's easiest to stick with that; penetrating stains can't go over a film. Also, water-based stains will cover oil-based ones, but not the other way around.
What kind of wood do you have?
Soft woods, like pine and cedar, easily accept any penetrating stain. But only certain kinds of seed oils, such as tung or linseed, will soak into ultradense hardwoods like ipe and mahogany. Always test these woods first to make sure the finish will penetrate and dry.
How worn is it?
On older decks, unless you can sand off the top layer you'll get the best protection from a penetrating finish: a clear toner, a semitransparent, or a semisolid. Solid-color stains are best on new wood.
How much grain do you want to see?
The less pigment a stain has, the more grain you'll see, as shown on the stack of blocks at left. But the more pigment, the longer you can go before recoating.
Where is it located?
Stain on horizontal or south-facing surfaces gets more punishing UV rays than on vertical surfaces or shady north sides. To minimize maintenance, consider stains with more pigment.