I grow it every year in my garden. I have two ENORMOUS plants that I need to split. Last year I split them and killed one ..
Lemongrass has a distinctive lemony, woody, taste. Not really like lemons but bright and citrus. You'll know it's taste if you've had Thai food.
It comes in a stalk. Much like a leek, there is a white end part trailing off to tough green leaves. Only the white part is edible, really, but you can use the leaves to flavor broths. Even the white part is very fibrous, so if you are going to eat it, it needs to be finely minced. Often lemongrass, like kaffir lime leaves, is used to infuse something with flavor and then removed.
It's a common ingredient in Thai curry pastes. A common ingredient in many southeast asian dishes.
Dry lemongrass is horrible, IMO. It's nothing at all like fresh and totally worthless as a substitute.
You can freeze fresh lemongrass. That's what I do. I grow a lot of it and freeze it for the winter. I am lucky enough to live near a huge asian market that sells it dirt-cheap year round, though.
You can also take a stalk of lemongrass and stick it in water and root it and grow it, but it needs very warm weather.
If you can find it fresh or frozen, I'd look for that. I would avoid a squeeze tube, if I could, because the lemongrass is mixed with something to make it form a paste.
You can find lemongrass in pretty much any asian market that carries produce. Whole Foods sells it too.