Gasoline is produced in oil refineries. Material that is separated from crude oil via distillation, called natural gasoline, does not meet the required specifications for modern engines (in particular octane rating; see below), but will form part of the blend. The bulk of a typical gasoline consists of hydrocarbons with between 5 and 12 carbon atoms per molecule. Many of these hydrocarbons are considered hazardous substances and are regulated by OSHA. The MSDS (Material Safety Data Sheet) for unleaded gasoline shows at least 15 hazardous chemicals occurring in various amounts from 5% to 35% by volume of gasoline. These include big names like benzene (up to 5% by volume), toluene (up to 35% by volume), naphthalene (up to 1% by volume), trimethylbenzene (up to 7% by volume), MTBE (up to 18% by volume) and about 10 others. Ref: (Tesoro Petroleum Companies, Inc. [5]) The various refinery streams blended together to make gasoline all have different characteristics. Some important streams are: Reformate, produced in a catalytic reformer with a high octane rating and high aromatic content, and very low olefins (alkenes). Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker, with a moderate octane rating, high olefins (alkene) content, and moderate aromatics level. Here, "cat" is short for "catalyst". Hydrocrackate (Heavy, Mid, and Light), produced from a hydrocracker, with medium to low octane rating and moderate aromatic levels. Natural Gasoline (has very many names), directly from crude oil with low octane rating, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes) and zero olefins (alkenes). Alkylate, produced in an alkylation unit, with a high octane rating and which is pure paraffin (alkane), mainly branched chains. Isomerate (various names) which is made by isomerising Natural Gasoline to increase its octane rating and is very low in aromatics. (The terms used here are not always the correct chemical terms. Typically they are old fashioned, but they are the terms normally used in the oil industry. The exact terminology for these streams varies by oil company and by country.) Overall a typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The exact ratios can depend on the oil refinery that makes the gasoline, as not all refineries have the same set of processing units. the crude oil used by the refinery on a particular day. the grade of gasoline, in particular the octane rating. Currently many countries set tight limits on gasoline aromatics in general, benzene in particular, and olefins (alkene) content. This is increasing the demand for high octane pure paraffin (alkane) components, such as alkylate, and is forcing refineries to add processing units to reduce the benzene content. Gasoline can also contain some other organic compounds: such as organic ethers (deliberately added), plus small levels of contaminants, in particular sulfur compounds such as disulfides and thiophenes. Some contaminants, in particular thiols and hydrogen sulfide, must be removed because they cause corrosion in engines. Search Gasoline at Wikipedia.org

Simple answer: By distilling it in the absence of air. If you start heating a barrel of crude oil, the first thing that evaporates will be gasoline vapor, which you can pass through cooling pipes so that it re-condenses. After that come kerosene, light oils, heavy oils, and then asphalt. Through a great amount of scientific research, chemists have discovered ways to get more gasoline out of oil than just the process I have described.