Typewriters are mechanical -- sometimes electromechanical -- writing devices. They have various designs, but all have keys that you press to leave marks on a sheet of paper or similar medium. There have been many different types of typewriters during the history of the devices, from early dome-shaped models to more recent electric typewriters. However, most standard mechanical typewriters share certain components in common.
The carriage is a box-like structure that contains the mechanisms that hold and position the paper, as well as the paper itself. As you type, a device called an escapement moves the carriage from right to left in precise steps, advancing the page along so that the letters are typed in sequence. The carriage return lever allows the carriage to move back to the beginning of the line.
The platen is a large roller, typically made from rubber or a rubber-like material. It is positioned horizontally in the carriage. The paper wraps around the platen, which supports it and provides a firm surface for the typebars to strike against. At the end of a line, as well as moving the carriage itself, the carriage return lever rotates the platen by a fixed amount to move the paper up so the next line can be typed. The feed rollers position the paper against the platen.
Typebars and Typebasket
The typebars are the thin, movable metal arms that are used to imprint marks on the paper. They typically have two characters, which are molded into the metal in reverse. For the letter keys, the top character is the uppercase form of the letter while the lower character is the lowercase form. Number keys use less common punctuation marks or fractions as the secondary character. The typebars together with the mechanism that links them to the keys are called the type basket.
The keyboard consists of the keys which you use to type. Most of the keys are letters, numbers or punctuation marks. The shift key is a mechanism which literally shifts either the carriage or the typebasket so that uppercase letters and other characters can be typed; these are typically placed on the upper part of the typebar, making it necessary to reposition either the typebars or the paper for them to be typed. Because the shift key takes some strength to operate, there is a shift-lock key so you don't have to hold down the shift for long periods. The space bar moves the carriage along without typing anything, to leave a space between words.
Keytops are the coverings, typically round or square, placed on typewriter keys to make typing more comfortable and also to indicate what letter or character is being pressed. Early models generally had keytops made from metal with paper letters protected by a glass or celluloid disc. As technology progressed, keytops came to be made from plastics.
Ribbon and Ribbon Spools
The ribbon is a thin strip of fabric or polymer such as Mylar, which is covered in pigment. It is positioned on two spools, with the ribbon running in front of the paper. When the keys are pressed, the typebars are pushed against the ribbon, leaving the imprint of a character behind. Typewriter ribbons are typically black but other colors are possible, as are bi-colored ribbons with (for example) a red and a black edge.