Ah, yeah. They last for 6 minutes and give sight at the location you drop them. Extremely useful to keep tabs on the enemy so you know where it is safe to farm and who's vulnerable for ganks. Also, you can put them in the river to watch over the rune spawns to make sure your team has control over them.
There's also true-sight wards that will show invisible units, but these have almost no vision on their own (and don't last as long) so these are used either to find enemy wards and destroy them, or to counter invisible heroes like Riki.
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No amount of planning will ever replace dumb luck.