How do companies that I've applied to for a job search my employment history?

What kind of program or service do they use to search for an applicant's former employer information? Do they only call the employers listed on your application? There are a couple short-term employers that I've had in the past that I don't ever list because I didn't leave on good terms (I quit without notice). I used to work in HR and we only called employers on the application, we didn't do indepth searches. I'm just wondering how corporate employers do their background checks for employment history.

How do companies that I've applied to for a job search my employment history?
I actually use Lexis/Nexus to conduct background searches on prospective hires. Since I am in a Corporate environment (I am the HR Director) we do thorough background checks. What is also important to us is if whether or not the applicant is telling the truth regarding there last salary which is included in the report. One applicant in particular who we were going to hire stated he was earning 65K at his last place of employment. When we received the report he was actually earning 28K. Be advised these types of searches are conducted for higher level positions or those positions where there is a security issue (access to our most confidential systems.) I do not perform these types of background checks for the lower level positions. Depending on the types of positions you are applying for HR may play a role.