This contains four fonts: "Unifont", "Unifont Upper", "Unifont CSUR", and "Unifont Sample".
unifont.ttf is a bitmap font converted into a scalable TrueType outline font, Unifont. Each pixel in the original bitmap font is represented as an outlined square. The font provides a glyph for each visible code point (character) in the Unicode Basic Multilingual Plane (BMP, or Plane 0). The BMP contains most of the world's modern writing scripts. This font looks best at 12pt.
unifont_upper.ttf is a bitmap font converted into a scalable TrueType outline font that covers Unicode ranges above Plane 0. Most of its glyphs are in Plane 1, the Unicode Supplemental Multilingual Plane (SMP).
unifont_csur.ttf is a bitmap font converted into a scalable TrueType outline font that contains some scripts in the ConScript Unicode Registry (CSUR). These scripts are not part of the Unicode Standard, but are a popular use for the Private Use Area (PUA). This font contains glyphs in Plane 0 and in the higher PUA planes.
unifont_sample.ttf is an SBIT font that contains combining circles and is therefore suitable for illustrating individual Unicode glyphs. The other font files do not contain combining circles and so are suitable for general-purpose writing.
Complex fonts (such as Indic or Semitic scripts, where letters change shape depending on their position in a word, or such as Mongolian, which can be written vertically) will not render perfectly. The philosophy behind this font, though, is that anything meaningful is better than an empty box for an unknown glyph.