Some car repair items can be purchased from cheaper unauthorized retailers (OES and OEA), but certain critical items like timing belts must be bought from the manufacturer-specified companies (OEM).
If you have purchased a good model autombile, most of the time you have spent maintaining it probably has been changing the oil, oil filter, air filter, and the normal fluids that require replacement. Most of these items are not specific and have some level of tolerance to varied brands or versions. In my 1991 Honda Accord, for example, I have replaced the stock airbox, air filter, and intake tube with a larger aftermarket model from AutoZone to reduce the restriction and increase horsepower (you can do this on cars without a lot of electronics; newer cars have computers that are responsive to changes in intake airflow and temperature and get a bit ticky when you change things). Fluids that require specificity, however, are brake and clutch fluids; if you use DOT3 when you were supposed to use DOT4, it will often cause problems. For the most part, though, replacing fluids and items designed to wear out is a routine process in which you can choose from several brands (oil is the primary example here).
When you have to conduct repairs on your vehicle and utilize replacement parts, you must be very careful about which parts you choose to purchase and from what manufacturer you buy them from. If you allow mechanics from the dealer of your make to repair the car, you don't have anything to worry about, but you should be careful about going to a random auto shop or purchasing the parts yourself if you are a DIYer.
The industry has developed lingo to guide consumers, but it can be a bit confusing. OEM refers to Original Equipment Manufacturer, and it has two specific meanings. The first is a company that makes air filters and then sells them to Honda, who puts its own brand name on them and resells them for its cars. The second and more recent definition refers to an incorporation of a product acquired by a company from another company into a new product of its own brand. Usually, only the first case applies to car parts, but in either scenario, you can rest assured that a mark of OEM on a product means that it is manufactured and intended specifically for application in your car. OEM parts are more expensive, but for certain parts (discussed later) it is absolutely necessary to ensure that they are OEM. If you can't tell if what you're buying is OEM, ask!
The two terms that you have to be concerned about are these: OEA and OES. These stand for Original Equipment Aftermarket and Original Equipment Supplier. Both of these terms mean that the products are coming from a manufacturer that is not an authorized manufacturer for the car company. This, however, does not necessarily mean that the parts are bad.
Some parts that are fine to use OEA or OES for are as follows: engine mounts, tires, radio, air and oil filters, alternators, fuses, doors, and basic hinges and other mechanisms. Think of it this way: if the item breaks, will it cause critical damage to your car? If not, then it is probably fine. The good thing about OES or OEA parts is that they are often cheaper, so if you find that they are not as reliable, you are not out a lot of money.
Some parts that are absolutely vital to use good OEM parts for are the following: timing belts, balancer belts, tensioner springs for these belts, electronics and switches, water pumps, CV joints or ball joints, and other parts critical to the car's operation. The reason is because the OES or OEA manufacturers often don't apply the same manufacturer-specific tolerances for those items. Yes, you pay for the difference in OEM parts, but if a timing belt breaks because you wanted to save 30 dollars, you have a much bigger than 30 dollar problem! Again, if you are unsure what parts you can save some money on, don't be afraid to ask the parts guys at your local dealer.