A rock is a bundle containing a specification file (called a "rockspec") and files providing Lua modules.

A rockspec is a Lua file containing a series of assignments to variables that provide various information about the rock, such as description metadata, dependency relations and build rules. Rocks are created from rockspecs.

When packed, a rock is an archive file in ZIP format, with the .rock filename extension. When installed, a rock is unpacked into a directory in the local rocks repository.

There are several types of rocks, and when packed they are identified by their filename extensions. These are:

• Source rocks (.src.rock): these contain the rockspec and the source code for the Lua modules provided by the rock. When installing a source rock, the source code needs to be compiled.
• Binary rocks (''.system-arch''.rock: .linux-x86.rock, .macosx-powerpc.rock): these contain the rockspec and modules in compiled form. Modules written in Lua may be in source .lua format, but modules compiled as C dynamic libraries are compiled to their platform-specific format.
• Pure-Lua rocks (.all.rock): these contain the rockspec and the Lua modules they provide in .lua format. These rocks are directly installable without a compilation stage and are platform-independent.