Half-Life: Source is a straight port, lacking any new content or the Blue Shift High Definition pack. However, it does take advantage of vertex and pixel shaders for more realistic water effects, as well as Half-Life 2's realistic physics engine. They also added several other features from Half-Life 2, including improved dynamic lightmaps, vertex maps, ragdolls, and a shadowmap system with cleaner, higher resolution, specular texture and normal maps, as well as utilization of the render-to-texture soft shadows found in Half-Life 2's Source engine, along with 3D skybox replacements in place of the old 16-bit color prerendered bitmap skies. The Half-Life port possesses many of the Source engine's graphical strengths as well as control weaknesses that have been noted in the Source engine. Half-Life: Source is available with special editions of Half-Life 2, or separately on Steam.
Half-Life: Source has been criticized for not fully utilizing many of the features of the Source engine found in Half-Life 2, as it still uses textures and models from the original game. Due to this, a third-party mod remake called Black Mesa is also under development.
Source: Wikipedia, "Half-Life (video game)", available under the CC-BY-SA License.