What is the difference between points and pixels?
A pixel is a single square 'picture element' (hence pix-el), i.e. a single dot in your image. A 10x10 image is made up of a set of pixels in a grid 10 wide by 10 high, totaling 100 pixels.
The 'point' (pt) on the other hand is a unit of length, commonly used to measure the height of a font, but technically capable of measuring any length. In applications, 1pt is equal to exactly 1/72th of an inch; in traditional print technically 72pt is 0.996264 inches, although I think you'll be forgiven for rounding it up!
How many pixels = 1pt depends on the resolution of your image. If your image is 72ppi (pixels per inch), then one point will equal exactly one pixel.
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Point is a physical unit of length, used in typography. It's equal to 1/12 Pica, and 1 Pica = 1/6 inch. So 1 pt = 1/72 inch.
Therefore, on a 72 ppi display, 1 point = 1 pixel.
A pt is 1/72 of an in, and a px is 1/96 of an in.
A px is therefore 0.75 pt [source].
In CSS, everything is somewhat abstracted, so a unit such as a "pt" is not necessarily one point in physical size, especially on a screen, an "in" is not necessarily one inch in size, and so forth. Even a "px" is no longer necessarily one pixel in size anymore: Everything is scaled to be consistent with a hypothetical 96 ppi device viewed at normal reading distance, meaning that on screens that differ significantly from 96 ppi or from normal reading distance, everything will be scaled, but still maintain the same relationships ie a pt will still be 1.33334 px units and still be 1/72 of an in unit.
In print, a point was traditionally somewhere from around 1/67 of an inch to 1/72.5 of an inch.
In digital mediums, it has become a de-facto standard for a point to be exactly 1/72 of an inch nowadays, though there are still alternative measurements in less common use which vary slightly from 1/72, but not by much.
In print, you don't usually measure in pixels, because they are a technical detail about the target printer or device that are not an absolute measurement. For instance, a design may be printed at 125 dpi, 300 dpi or at 1200 dpi and still be the same physical dimensions.
A point is a typographic meassure, that means it is a physical meassure of length, like miles, inches, meters or an astronomical unit. Historically, the length of a point varied from different locales and cultures, but with the rise of desktop publishing and internationalisation the following convention has established:
A pixel is the smallest unit of digital image data. That is to say a pixel is without actual physical size. Pixels are used to display an image on screen or print it, converting the image information in pixels to physical representation. Screens have their pixel density measured in ppi (pixels per inch), whereas printed images are measured by dpi (dots per inch) - for both the same amount of image pixels may result in hugely variable physical sizes, e.g., a 100X100 pixel image will be huge displayed on a outdoor advertising screen, or tiny when printed on paper at 300 dpi.