Sega Genesis/Mega Drive
The regular genny without the CD or 32x add-ons:
screen size: 320x224
Sprite pixels per scanline (this includes transparent pixels): 320
Sprites per scanline: 20
Sprites per screen: 80
screen size: 256x224
Sprite pixels per scanline (this includes transparent pixels): 256
Sprites per scanline: 16
Sprites per screen: 64
Same as normal modes, except the vertical resolution is doubled, for the tiles and sprites both. This makes the pixels appear wide. Tiles in this mode are 8x16 instead of the regular 8x8 px. This will take the same screen space as regular mode 8x8 tile. Using these double tiles will eat up ram that much faster, so the number of unique tiles is halved. Some emulators will not display this mode correctly.
Master System mode
Compatibility mode for Master System games, see the Master System post for more details.
Global palette is 9bit RGB ( https://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_palettes#9-bit_RGB
You can use 4 palettes of 16 colors each, the first index of each palette is considered transparent. The palette is applied per tile or sprite. One border color can be picked out of these 4 palettes, and it will be displayed for screen space out of the regular resolution, and also in case every pixel on the same spot is transparent. Picking one of the transparent indexes from the 4 palettes as this border color will show that particular color.
The genny can display shadow and higlight colors. The mechanics are a bit convoluted, so I'll just quote the original source
"This mode is kind of a mess. Essentially, tiles are either "shadowed" or "normal" based on whether or not their priority bit is set. Note that a transparent high priority pixel can make the opaque low-priority pixel below it "normal" rather than shadowed. On top of this, sprite colors $3E and $3F will modify the shadow highlight state of the pixels below them rather than displaying as normal, allowing you to set shadow or highlight status on a per-pixel basis. There are 15 distinct voltage levels for each channel. Normal mode maps the 8 values evenly over this range. Shadow scrunches them down to the bottom half and highlight pushes them up to the top half. There is exactly one level that overlaps between shadow and highlight."
The shadow and highlight palette taken from sega retro: http://segaretro.org/images/2/28/Mega_Drive_Palette.png
Sprites are either 8x8, 16x16 or 32x32 pixels in size. It is allowed to mix sprite sizes.
The video ram can be configured as you wish more or less, with a regular count of 1296 tiles available for backgrounds. With on the fly loading from the cart and configurable ram allocation, the number of unique tiles is more or less a non issue. The tiles can have a flip by x and/or y flag set, so they can be flipped vertically and/or horizontally to save ram. Most carts are 512kb to 2Mb. Largest old commercial cart is 5Mb (Street Fighter 2), while today some use larger carts, like Pier Solar (8MB).
The genny can display 2 BG layers, 1 sprite layer and one window layer. The window layer substitutes the BG layer A where shown. It is non scrollable thus usually used for status bars and such. Each tile and sprite has a priority flag that can be set or unset, so it is possible with some scene arrangements to have the screen show what appears to be multiple background planes. If you look at some genny game videos, you can see that the different scrolling "planes" are actually only 2 BG layers that are separated from each other so that different parts of the same BG layer don't overlap.
BG layers can be scrolled at different speeds horizontally by pixel (those old skool wavy effecdts) or by tile. With some scene arrangements, this can be used to simulate many backgorund layers and fake 3d. Vertically, the BG layers can only be scrolled at different speeds in 16px (2 tile) increments. There is no hardware supported rotation.
You can have raster splits on the genny, and change palettes on the fly to show more colors on screen at once, but the raster split has artifacts, so this is usually masked by sprites (sonic 1 water level is a good example).
As a side note, the genny has 2 gamepad ports, and a standard gamepad has a d-pad and 4 buttons (A B C + Start). Later on the genny got a controller with more buttons (I suspect Street Fighter 2 had something to do with this), which has a d-pad, 6 main button ( 2 rows of 3 buttons named A, B, C, x, y, z), a right shoulder button named mode and a centrally positioned START button. I'm not sure about light guns, wheels and multi taps, but I'm certain some games (Pete Sampras Tennis) had 2 extra gampad ports on the cart itself. Some sonic games used the so called "lock-on" technology which enabled players to plug a cart into a cart for extra features.
Take care not to use the top and botom row of tiles for anything important, the same as the first and last 2 column of tiles in any given screen, due to some TV screens cutting them off at some point or shifting the screen horizontally by some margin of pixels, depending on the TV manufacturer and the TV standard (PAL/NTSC).
Here's a link to a youtube video showing off some of the genesis more impressive gfx feats: https://www.youtube.com/watch?v=MjkDq02Apyw