This is a Black Widow Spider...and so are these:

 This is a Latrodectus mactans or Southern Black Widow spider - the most prevalent of the Black Widow species found in the United States.  This Black Widow has a body that is about a half an inch long. This Black Widow spider is nocturnal and hangs upside down in the web, so that the warning markings are clearly visible.  Note the hourglass.

This is the Northern Black Widow, Latrodectus variolus, which is found primarily in the northern states, from New England to Florida thru Texas, Oklahoma and Kansas.  This spider's markings vary.  Sometimes the body color will be dark brown and the bright dots can be yellow to red and may be anywhere on the bulbous part of the body.  The most important thing to remember is - err on the side of caution.

The Western Black Widow, Latrodectus hesperus, resides in West Texas through Oklahoma and Kansas on north to Canada,and west to the Pacific Coast.  There is anecdotal evidence that this spider can get much larger than the half inch body reported in the literature.  None of the Black Widows are very large (one and a half inches, including the legs), but this variety is the largest with the longest legs in relation to the body.

 It is worth noting that there is a new species of Widow spider in the United States. This is the former tropical resident Brown Widow spider. The body is similar in form to a Black Widow, but it is tan to brown with varying spots and rings. The Brown Widow female does have red or orange spots that sometimes merge to resemble an hourglass, but these occasionally occur on the back of the bulbous part of the body and not exclusively on the abdomen. As with the Black Widow, the adult female is the dangerous biter. The Brown Widow spiders' habits and temperament are thought to be similar to the Black Widow but their range would be restricted to the warmer regions of the country.   The bite of the Brown Widow is thought to not be as dangerous as the Black Widow.  The venom is actually more toxic but the amount injected is nothing like the amount a Black Widow is capable of injecting.  The area affected by the bite then remains localized and is generally not any more dangerous than an average house spider bite.