Internal shading is almost always adjustable or retractable, and is typically in the form of roller or venetian blinds or curtains. It is easily adjusted and maintained, and sometimes can provide night-time blockout. It is also generally cheaper and is particularly effective at controlling diffuse and reflected light, the major causes of glare. Curtains and blinds can moderate brightness, and venetians and louvers can redirect the light.
More info on The Effective Control of Light and Glare
Internal devices do not stop all the direct sunlight until it has passed through the glazing: that means that the solar energy enters as short-wave radiation which will be absorbed by walls, furniture and floors; once the short-wave radiation has been absorbed, it will be long-wave infrared radiation, which means there is a heat build-up in the room. However, highly reflective internal shading may be also thermally efficient. If properly adjusted, venetian blinds admit diffuse light while excluding direct sunlight, and can also act as a daylighting device by redirecting light onto the ceiling.
The most efficient use is to combine internal shading with automation. In housing it is not the standard yet, but it may prove cost-effective in large office buildings.