A typical home inspection will cover the roof, exterior, attic, electric, plumbing, HVAC, chimney, garage, crawlspace, and basement. Most home inspectors include ancillary services more septic systems, detached buildings, pools, etc...

Most home inspectors are looking for material defects. InterNACHI, the International Association of Home Inspectors, defines a material defect as "A material defect is a specific issue with a system or component of a residential property that may have a significant, adverse impact on the value of the property, or that poses an unreasonable risk to people." (The link is to InterNACHI's article that defines material defects.) 

Most home inspectors will perform a visual inspection in search of these material defects. Additionally, they will report on conditions that are not material defects but are still a cause for concern. For example, most homebuyers would like to know if the gas water heater is 11 years old since their life expectancy is 10-12 years. 

Not all home inspectors are created equal. Some focus on delivering a good, quick inspection to ensure there are no major issues and the real estate transaction is completed. Others offer very high-quality, lengthy, and detailed inspections so that homeowners and buyers are familiar with every aspect of their home's health. Usually, these guys and gals are more expensive and are not recommended by real estate agents. (Agents are busy, and 6-8 hour inspections often push their limits.) 

Most inspectors lie somewhere in between, and no two inspectors are the same. Each has their background, expertise, and focal points. It is up to the client to decide which inspector is right for their needs.