A typeface is a particular set of glyphs or sorts (an alphabet and its corresponding accessories such as numerals and punctuation) that share a common design. For example, Helvetica is a well known typeface. A font is a particular set of glyphs within a typeface.
So, 12 point Helvetica is a font, and 10 point Helvetica is a separate font. The same goes for different weights – a 14 point Helvetica Bold is a different font than a 14 pt Helvetica Light. They are different fonts, but the same typeface.