Originally, Alpine Linux began as a fork of the LEAF project. The members of LEAF wanted to continue making a Linux distribution that could fit on a single floppy disk, whereas the Alpine Linux wished to include some more heavyweight packages such as Squid and Samba, as well as additional security features and a newer kernel. One of the original goals was to create a framework for larger systems; although usable for this purpose, this is no longer a primary goal.
Package management: Alpine uses its own package management system, apk-tools, which originally was a collection of shell scripts but was later rewritten in C. Alpine currently contains most commonly used packages such as GNOME, Xfce, Firefox, and others.
Running from RAM: Alpine Linux can be installed as a run-from-ram distribution. The LBU (Local BackUp) tool optionally allows all configuration files to be backed up to an APK overlay file (usually shortened to apkovl), a tar.gz file that by default stores a copy of all changed files in /etc (with the option to add more directories).
Security: PaX and grsec are included in the default Alpine Linux kernel, which aids in reducing the impact from exploits similar to the vmsplice() local root exploit. All packages are also compiled with stack-smashing protection to help mitigate the effects of userland buffer overflows.
Size: the base system in Alpine Linux is designed to be only 4-5MB in size (excluding the kernel).
Alpine Configuration Framework (ACF): While optional, ACF is an application for configuring an Alpine Linux machine, with goals similar to Debian's debconf.
C standard library: Alpine Linux used uClibc instead of the traditional glibc most commonly used. Although it is lighter weight, it does have the significant drawback of being binary incompatible with glibc. Thus, all software must be compiled for use with uClibc to work properly. Recently (as of April 09 2014), Alpine Linux switched to musl, which is at least partially binary compatible to glibc.