Softshell is, as the name clearly suggests, a soft shell. Strictly speaking, we define a softshell garment as a garment without a membrane. So, softshell is the opposite of shell clothing. Softshell garments are often made from lightweight materials that provide excellent protection against the wind, meaning that a good old-fashioned anorak made from cotton can be classified as a softshell garment, although modern softshell products are often far more technical.
Because softshell garments do not have a membrane, they breathe far better than shell garments, but are not waterproof. Softshell garments are generally water-repellent, but water will completely penetrate the garment at some point in heavy or prolonged rain.