Here’s the short version for those in a hurry (click on the hyperlinked text to get the complete answer directly from the census bureau with a cherry on top!):

Census tracts are subdivisions of a county.  Although these boundaries are fairly stable, they do change from one census survey year to another.  A census tract contains about 4,000 inhabitants. 

What info can I get at the census tract level?  The only data being reported at the census tract level is information contained in Summary Files 1-4 of the Decennial Census (the last one was conducted in 2000).  Click on this link to get to the Decennial Census. 

Here is the Census Bureau’s description of the information contained within Summary Files 1-4 of the Decennial Census: 

“Summary File 1 presents counts and information [age, sex, race, Hispanic/Latino origin, household relationship, whether residence is owned or rented] collected from all people and housing units.

Summary File 2 Population and housing characteristics iterated for many detailed race and Hispanic or Latino categories, and American Indian and Alaska Native tribes.

Summary File 3 presents detailed population and housing data (such as place of birth, education, employment status, income, value of housing unit, year structure built) collected from a 1-in-6 sample and weighted to represent the total population.

Summary File 4 contains tabulations of population and housing data collected from a sample of the population. The data are shown down to the census tract level for 336 race, Hispanic or Latino, American Indian and Alaska Native, and ancestry categories.”

For directions on how to navigate through the Summary Files when you have a specific census tract number, go here.

For directions on how to obtain census tract numbers for a state, county, etc. go here

Hope this helps!  Leave a comment if you have anything to add or need me to find something else.