An female, adult redback is black/brown with a red striped down her back. Sometimes this may have breaks or be uneven, but not usually. She also will have an "hourglass" marking on her underside, like the black widow (they are closely related).
A juvenile female is black/brown and has the same markings, although the stripe may be broken (or even missing if she is very young!) and will often have a white outline around the stripe. Sometimes she will have white stripes on her legs.
A male is brown, inconspicuous and tiny, sometimes with some faint stripes.
The best way to find out is just do a Google image search for "female redback spider"
thorn spider
The Theridion Grallator, or happy face spider, is found on the islands of Oahu, Molokai, Maui and Hawaii. It is believed the spiders developed these markings to keep birds from eating them.
It is the marbled golden orb weaver.
It could be a black widow spider if it is small and thin-legged with a shiny round black abdomen and red markings under it. If it's fuzzy with a red spot on it's back, and the front part of the body is more-or-less the same size as back part, then it's a black jumping spider.
It can just be a red back spider...
maybe it's a black widow?
From the description, this sounds like an Arrowshaped Micrathena spider. It's a variety of the orb weaver and is known for its bright colors and spikes.
a wolf spider. they hurt but are not deadly to humans
A spider.
A wolf spider variation.
The baby black have a similar shape as the adult, however there is a vast colour difference. The immature Widow Spider is much smaller than the adult and is yellow/brown in colour. The characteristic hourglass marking on its back is yellowish and not red. It is only after maturing and mating that the female spider takes on the sinister black body with red hourglass colouration on its back. The young juvenile black window spider does not have any red markings.
The spider that is black with a white crescent on its back is the Brown House Spider. This spider is often confused with the red-back spider.