Logs provide critical information for evaluating oil and gas reservoir potential, as well as giving engineers and geologists a better understanding of what to expect during production operations.

From the Schlumberger Oilfield Glossary, a wireline log can be defined as:

A continuous measurement of formation properties with electrically powered instruments to infer properties and make decisions about drilling and production operations. The record of the measurements, typically a long strip of paper, is also called a log. Measurements include electrical properties (resistivity and conductivity at various frequencies), sonic properties, active and passive nuclear measurements, dimensional measurements of the wellbore, formation fluid sampling, formation pressure measurement, wireline-conveyed sidewall coring tools, and others.

Wireline logs rely on physical measurements to be made by instruments lowered into the borehole on a cable (wire).  Once the zone of interest is reached, the actual measurements are  traditionally recorded as the instrument is pulled up towards the surface.  There are many types of sophisticated wireline logging tools, including newer tools that allow for logging to occur while drilling (LWD). For a good overview of the various tool classes go here. 

Where can I find wireline logs for Colorado wells?

Publically available wireline logs (both raster and LAS format) are available through the Colorado Oil and Gas Conservation Commission website.  Both a direct database query or map search utility are available.  Additionally, there are commercial vendors that maintain raster and digital log databases for purchase.