Red maple is a separate species (Acer rubrum) distinct from the Japanese maples, Acer palmatum ctvs. It is a far larger tree, rather rapidly growing with largish leaves with three, sometimes 5, rather undefined lobes, often longer than they are wide. Native to the eastern US and widely grown for fall color. And most cultivars feature green leaves as opposed to red - the 'red' comes from the flowers, which appear before the tree leafs out.

'Atropurpureum' and 'Bloodgood' can be very similar in appearance - 'Bloodgood' is a named form of atropurpureum that has a very intense, dark red leaf that retains its color well throughout the season. Atropurpureum can be rather variable in leaf color with a sometimes washed out red tone that fades to a muddy reddish green in summer. Both tend to be relatively fast growing as J. maples go as well as some of the largest forms. Leaves of both will have 5-7 distinct lobes, very deeply cut compared to red maples, and serrate edges.

Mixed seedlings are also sold as 'Bloodgood'. The true item is somewhat distinctive, with a tall habit and comparatively deeply lobed leaves.

Vertrees(Gregory), Japanese Maples (Timber Press) has detailed descriptions.

It is also notable to mention that 'Bloodgood' is a grafted (cloned) variety of Acer palmatum. This is done because the seeds from the tree are sometimes green and sometimes red (usually not as bright however).

Certain varieties of Acer rubrum such as 'Autumn Flame', 'October Glory', or 'Red Sunset' are also propagated similarly to that of Japanese maple varieties, although I think they are usually cloned by cuttings. This ensures that the trees exhibit bright fall colors since seedlings of this species can be different shades of yellows/oranges/reds.

It is important that you check the tags of the trees that you consider buying to see what cultivar it is (and IF it is a cultivar). Plant names are always indicated by the following: Genus species 'Cultivar Name'.

And if you already knew this, I'm sorry I just thought a complete explanation would be helpful.

To clarify the actual difference in the trees, i've included some links to images:

Acer rubrum:
http://homepage2.nifty.com/chigyoraku/rubrum11.jpeg

Acer palmatum 'Bloodgood':
http://www.biol.uni.wroc.pl/obuwr/archiwum/3/image/17.jpg
http://www.sperlingnursery.com/images/Fall_Bloodgood.jpg

Acer palmatum atropurpureum:
http://pics.davesgarden.com/pics/Todd_Boland_1100141150_575.jpg
http://www.bonsaiglobal.com/generos/acer/atropurpureum.jpg

As you can see the acer rubrum has very different leaf shape, and it has green leaves in the spring and summer.

The images of the bloodgood don't do it justice, as mine has much brighter colors in early spring and fall. However, it also becomes somewhat greenish-bronze in late summer.

As you can see from the second atropurpureum link, the seedlings can lose there color quite easily. Also, the leaf shape varies quite a bit with atropurpureum.
Note also that there was once a cultivar of 'Atropurpureum', but the name now refers to all red seedlings. And as bboy mentioned, some people are IMPROPERLY labeling red seedlings as bloodgoods.

So, if you wanted a mid sized red tree for most of the year, a real 'Bloodgood' is a good choice.

If you want a large green tree with red fall color, some of the Acer rubrum cultivars listed in the above post would be a good choice.

If you want a cheaper red tree for most of the year, an atropurpureum might do the job. However, it might turn out to only have red fall color.