Differences in components
Different monitors differ in quality, and this is often dependent on the technology and components used internally.
Some CRT monitors use a single electron gun at the rear of the monitor to produce the electrons that will become the red, green and blue electron rays. However, higher quality monitors have an individual gun for each, which can increase picture quality.
The metal used for the mesh at the front of the monitor will also affect quality. Electrons also produce ions because of imperfections in the vacuum, and these electrons are destructive to image quality if they hit the phosphor. Consequently, meshes are made of relatively thick metal to prevent phosphor damage. However, in better quality monitors, a thinner, yet tougher metal alloy is used for the mesh. Because it's thinner, it means more light can get through, making for increased brightness and higher contrast.
Aperture Grille v Shadow Mask
Each CRT has a sheet of metal at the front of the monitor which (partly) defines the pixels on the screen. Shadow mask is an older technology, and is literally a piece of metal with millions of holes in it which allow the different cathode rays through to hit the phosphour. Because a shadow mask covers the whole back of the screen, protecting the phosphor from stray ions, it also limits the strength of the rays, reducing the brightness of the monitor.
Aperture grille is a newer technology which defines the gaps through which electrons pass using a mesh of wires rather than a sheet with holes in. Whereas a shadow mask is made of circular holes, the grill is made of vertical slots. Because it is by its nature thinner, it allows for brighter displays. However, the grill is fragile and prone to being knocked around. The grill is therefore strapped to the monitor using stiff horizontal wires - this is what causes the distinctive pair of lines across high-end aperture grille monitors.
Invar mask is a variant of shadow mask, and uses a thinner, stronger metal to form the mask, allowing for better image quality whilst remaining cheaper than aperture grill to produce.
Sony's Trinitron brand and Mitsubishi's Diamondtron brand are both variants of Aperture grill.
Dot pitch and resolution
Each pixel on the CRT screen is defined by lighting up combinations of the red, blue and green phosphors that make up the pixel. With a varying strength of electron gun operating on each phosphor, different colours are produced - with red, blue and green all fired on maximum strength, that means bright white is produced.
Dot pitch is measured on most monitors as the distance, diagonally, between two phosphors of the same colour. However, some manufacturers quote dot pitch on monitors as the horizontal distance between phosphors, which can make them appear better specified, on paper, than perhaps they are.
Dot pitch combined with viewable image area defines the maximum resolution of the screen. For example, if you have a 21" monitor with a viewable area of 401mm x 298mm, and a dot pitch of 0.26mm, you will have a CRT capable of displaying a maximum resolution of 1758 horizontally. How so?
Well, if we take 1 as the diagonal dot pitch, Pythagorus dictates that the horizontal dot pitch (ie the gap between pixels as rendered horizontally by the graphics card) will be 0.87. 0.26 diagonal dot pitch multiplied by 0.87 makes for a diagonal equivalent pitch of 0.228 horizontally. 401mm horizontal viewable screen area multiplied by 0.228 is 1758, hence a maximum of 1758 pixels are usable on the screen.
In general, the lower the dot pitch, the better the sharpness of the image.