The Intellivision -a portmanteau of "Intelligent Television"- was introduced in 1979. The Atari 2600 had come out in 1977 and Mattel decided it was high time to cash in on that sweet video game money, so they got to work and the Intellivision was born two short years later.
The Intellivision plays alright. I enjoy Burger Time, which is one of a small stack of games I own. The Intellivision tried to cash in on ports of games one couldn't play on the Atari, and whilst that's clever, it also means some good games aren't available and not all ports are great. That said, the Intellivision is a more powerful machine than the 2600. Its graphics are more sophisticated and its controls allow for more complex gameplay.
It is massive and heavy. It's got gold and woodgrain. This is a console from the seventies and it's not taking any prisoners.
125 games were developed for the Intellivision; it suffered, as did all, under the great video game crash of 1983. Mattel got burned bad enough that they didn't make another console for another 25 years.
What didn't help are these controllers, which were fixed to the device and could not be swapped out. Instead of a joystick they have a big, round wiggly button thing and the action buttons are on the side of the controller.
The number pad was the Intellivision's "thing". Games use overlays, adapting the pad to various actions. It was quite inventive, but there's really no doubt that the Atari 2600's more simple gameplay beats it.
Here's Space Battle with its two inlays.
The controllers have slots in the top where the inserts can be slid in; it looks pretty cool, but in practice, it's finicky. Especially in the case of Space Battle, which is the most confusing game since life itself.
The games insert in the side of the device, for some reason. What it means is that you can't really keep the Intellivision in a cabinet or anywhere where you can't conveniently get to the side - however, you're not likely to want to as the controller cords are quite short.
A computer component was made. It attaches to the side of the machine, turning it into a weak computer - read about it here.