Executive orders are not mentioned by the Constitution, but they have been around a long, long time. EO's and Proclamations are not law, but they have the effect of statutes. A typical modern Proclamation might declare a day to be in someone's honor. Historically, they have had broader effect, such as the Emancipation Proclamation . A typical EO might instruct the government to do no business with a country we are at war with.
Executive orders are subject to judicial review, and can be declared unconstitutional.
Today, EO's and Proclamations are sequentially numbered. The average president issues 58 EO's a year. Thanks to Richard Barr for the idea. Originally, there were six members, and the number has fluctuated up to as many as ten. In 1869, the number was set in the law at nine, and it has remained at nine ever since.