What's the difference between a boy and girl spider

In general, female spiders are much larger than the males. In some cases, may be 10 times (or more) larger than the male spider. The females (generally) live longer, as well.

Some species of spider appear to be exactly the same and differences are not easily detectable (as with some species of tarantulas). This requires picking up the tarantula(s) and investigating their limbs, their abdomens, and various other body parts.
(note: the tarantula may not like this)

Image and info Male vs Female Black widow spider:
(as you can see, there is a BIG difference in size = easy to tell which is boy vs girl)
http://www.brownreclusespider.org/black-…

Differences between male & female spiders/eHow
http://www.ehow.com/info_8281150_differe…

All about spiders:
http://www.spiders.us

All insect identification:
http://www.insectidentification.org

Female spiders have organs that can store sperm, fertilize eggs, and produce eggs.
Male spiders can produce sperm and have organs to inseminate female spiders.
In appearance, male spiders are often smaller than females of the same species, or, because of sexual dimorphism, can look entirely different.
male and female golden orb weaver:
http://pics.davesgarden.com/pics/2008/07…
Female southern house spider:
http://www.flickr.com/photos/32083154@N0…
Male southern house spider:
http://www.flickr.com/photos/32083154@N0…
male and female western black widow:
http://www.flickr.com/photos/32083154@N0…

What's the difference between a boy and a girl human?

If you can come up with the answer to the above, you will have answered your question as well.

There are thousands of different kinds of spiders and each one has a different way and some have no differences between sex so noone can answer this question unless you give a specific species of sider

One lives after sex and one gets eaten.