As others have already mentioned, it's a stinging nettle.
The hairs that deliver the sting into your skin are actually not very strong, and they mostly reside on the underside of the leaves. There's a saying "if you grasp the nettle firmly, it will not sting you", and this comes from the fact that if you choose to harvest a nettle, by grasping it in a smooth motion to use the topside of the leaves to protect you as you grip the stem to pull up the plant, you can avoid being stung.
There's herbal extracts you can get that include nettle, I've also consumed it in a tea (and it had quite a unique, yet pleasant taste). I have heard of it being used as a flavour ingredient in soup also.
The stings do indeed ease after about a day, but they can be quite painful and one can cause more pain overall by scratching them as they are intensely itchy. In my personal experience, once stung, it is important to ensure that you've removed all the stinging hairs from yourself and your clothes or they will continue to sting. As a child I was always advised to apply wet mud to the stings and let it dry in order to ease the pain and itching - it did seem to work.