MACRS, or the Modified Accelerated Cost Recovery System, is the IRS method for depreciation of assets. The IRS identifies assets and places them into categories that enable the company to get more depreciation during the early years and over a common useful-life period. The categories are very specific, while the useful life is determined by the IRS and does not necessarily correlate to real-world usage.
For example, the useful life of a computer in the business world is usually less than three years due to changing technology, but the IRS has determined that the depreciable life of a computer is five years, which is the same as a bus. Meanwhile, a semi-tractor trailer (the cab of the truck) has an IRS depreciable life of three years, and we all know that those last considerably longer in the real world! Don't worry about it. If you need to know the IRS method of depreciation, you will be using the IRS tables and referring to IRS documents.
Because the IRS will only allow depreciation to be calculated using certain methods, it is very common for corporations to maintain two sets of books, which is perfectly legal—if and only if one set of books is maintained using GAAP and the other set is maintained using IRS-required reporting methods.