Spiders rarely bite and do not go out of their way to attack humans or pets.  In most cases, spiders would rather run away than bite, and tend to bite as a last resort defensive measure.  When bites do occur, they tend to be mild (similar to a mild bee sting).  In rare cases, bites can cause more noticeable reactions.  If you have been bitten by a spider, collect the spider and get it to your local Extension office for identification.