Visual effects are the building blocks of demos. To impress the audience, an effect needs to look good, have some novelty in it, and be technically well-executed. Especially during the early years of the demoscene, in the late eighties, creatively pushing hardware was highly regarded, but later on visual quality became increasingly important. The capabilities of home computers have improved considerably during the last 25 years, which has let the programmers create effects that would have been way beyond the reach of the early 8-bits such as the Commodore 64. Likewise, old tricks have become outdated or unfashionable and fallen out of use.
Most demos released in the late 1980s or early 1990s featured some kind of scrollers: text sliding across the screen. Several variations such as sinus or parallax scrollers exist, but the basic idea is still the same. Scrollers were often used for informative purposes, like displaying the credits or for sending greetings to fellow groups. Other common effects of the time were colorbars, wobblers and sprite graphics moving on top of the background. Common to all of them is that they directly reflect the capabilities of the contemporary hardware. A bit later image-based effects, such as various tunnels and zoomers, started to gain ground, reflecting the move away from bitmapped graphics hardware, which was ill-suited for such feats.
Vector graphics such as rotating objects and scenes became popular in the early 1990s, first in the primitive flat shaded format. Throughout the nineties the sophistication of vector graphics increased little by little and at the end of the decade you could see texture mapping, real-time shading and bump mapping in countless demos. An important turning point was the migration to 3D accelerators starting from 2000: low resolutions and hand-crafted software-based 3D engines gave way to high-resolution hardware-accelerated rendering. These days vector graphics are so fundamental to demos that often all the effects of a production are implemented using them in one way or another.