Also known as the Advanced Package Tool, APT was first introduced in Debian 2.1 in 1999. APT is not so much a specific program as it is a collection of separate, related packages. More here
With APT, Linux gained the ability to install and manage software packages in a much simpler and more efficient way than was previously possible. Before its introduction, most software had to be installed either by manually compiling the source code, or using individual packages with no automatic dependency handling (such as RPM files). This could mean hours of “dependency hell” even to install a fairly trivial program.