Dave,
Karen is spot on. It sounds like you are just getting started, so here are a few tools you'll probably use all the time for research once you get the hang of them... Don't be overwhelmed by this, since it may be more info than you really need at this stage. If you just want a quick answer, skip to the G.I.S. and MUNICODE sections. Find out the zoning district on G.I.S., and then look up that district's zoning regulations on Municode. You could also even skip THOSE steps and meet immediately with the City, but if you spend the time and do the extra research, you will expand your toolbelt and learn a lot more about the property in the process.
TAX DATA
Start by Googling "pay tax [insert county name]." Somewhere on the county tax assessor, tax appraiser, or tax commissioner's website (each municipality is different), you will be able to find the the tax parcel ID number and other helpful information. The tax maps can also give you an idea of the property's boundaries, but they are often inaccurate. For the boundaries, rely on the legal description in the deed, or better yet get a survey and title work done, if (1) you decide you are very serious about the deal and (2) you determine that the cost is justified.
For leverage in your price negotiation, you can also see if they owe any back-taxes.
CLERK OF COURT
In Georgia, you can pay a very small fee for online access to the state clerk of court's database. You can look up property deeds and plats on their website, although some of the older stuff may not be online. Invaluable. Just to give you an example, in GA the website is www.gsccca.org. @John Weidner, do you know the Illinois website?
GEOGRAPHIC INFORMATION SYSTEMS (G.I.S.)
Increasingly, cities and counties make zoning and other information publicly available in online, interactive maps. It can be a very efficient way for you to get a lot of info quickly. Just be aware that sometimes there are bugs, and other times the information may be out of date. Also be aware that sometimes a parcel will have not just "base" zoning but also "overlay" zoning (additional regulations that are overlaid on top of the base zoning).
MUNICODE
Start by checking the municipality's website for their zoning code. If you can't find it easily, check Municode.com. It is a website that a lot of municipalities use to publish their zoning code online.
YOUR STATE'S "SECRETARY OF STATE" WEBSITE
Not really related to your question, but useful. You can look up ownership and contact info for corporate entities here. If a business owns a property you are interested in, this can help you figure out who is the wizard behind the curtain (to use a Wizard of Oz metaphor).
HOA COVENANTS
If the property is in a subdivision with a homeowners association or condo association, the HOA can set additional rules about what you can do with the property. HOA covenants are typically published online at the Clerk or Court's website.