The weekend is made up of the days of the week that are typically designated as rest, or non-working days. It occurs at the end of what is a typical workweek for the region, hence the name weekend. The weekend is the counterpart of the weekday and occurs on different days in different regions.
Weekends in the US and other countries
In the United States, as well as many other English speaking countries, the weekend is made up of 2 days, Saturday and Sunday. The other 5 days are Monday, Tuesday, Wednesday, Thursday, and Friday, which make up the weekdays. In other countries, the weekend may fall on Friday and Saturday instead.
Part of the reason is that the idea of a weekend has its origin in the Sabbaths (days of rest/worship) of various religions. For example, the Sabbath in Christianity occurs on Sunday; for Islam it occurs on Friday; for Judaism it occurs on Saturday. Regions like the US where Christianity and Judaism are more prevalently practiced religions tend to have weekends that fall on Saturday and Sunday. In regions where Islam is the predominant religion, their weekends tend to fall on Friday and Saturday, since Friday is the Muslim prayer day.
The modern concept of the weekend
Though weekends originated from the Sabbaths, the Sabbaths only afforded one day of rest per week, so people of the past used to have 6 or even 7 day workweeks. Weekends in their current form didn't really start taking shape until the Industrial Revolution in the 1800s, starting with factory workers in Britain.
In the US, the first 5-day workweek was implemented only in 1908, to accomodate Jewish workers so they wouldn't have to work on their Sabbath (Saturday), but it wasn't until 1940 that a 40-hour workweek was mandated, and two-day weekends were adopted throughout the US.