These printers are often used for receipt-printing. They use sprocket paper (one long, perforated sheet with holes on both sides), and ribbons to produce the text.
These printers are fairly inexpensive, but you cannot print graphics. They use single-ink ribbons (generally black), and as such, you should not expect full-colour printouts, either. They're ideal for receipts, first drafts of writing, and other, non graphical work. They're also fairly large, so finding space in your home or office might present you with some challenge if you have space constraint.

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