It can just be a red back spider...
The spider you are referring to is likely the black widow spider. The female black widow has a distinct red hourglass shape or white crescent on its abdomen. They are known for their venomous bite, so caution should be taken when encountering them.
It's probably a black widow and yes very poisonous.
a wolf spider. they hurt but are not deadly to humans
A spider.
I THINK THAT WILL BE Misumena vatia flower spider(female) http://upload.wikimedia.org/wikipedia/commons/7/7f/Misumena_vatia_female_Luc_Viatour_1.jpg
A black widow. This spider is only a black widow if the entire spider is black, with a round-ball looking back-body, female variety if there is a red hourglass shape marking on the underside of the spider. From the top black widows are completely black.
a red back.
A black widow spider is black with a red mark on it's abdomen; this might or might not be what you meant. The black widow sometimes has a red mark on her back, as well. The red backed spider of Australia, a relative of the black widow, has an orange red mark on the back.
red back spider
a spider.
red back