Minecraft wiki entry for Texture_Pack, associated text: The selected texture pack is saved to the options.txt file in the .minecraft folder, so the "skin:" value can be manually changed to the name of the .zip. This can be useful if the texturepack crashes the main menu, thus making impossible to change the pack using the texture selection screen. Simply erase the pack name and it is reset to default.
Resource packs that only add textures are called "texture packs" in Minecraft Marketplace.‌[BE only]
If the player's head is inside of a block while having a texture pack enabled, the visible block texture uses the one from the default texture pack.