Candela is a base SI unit that measures the Luminous Intensity of visible light emitted by a specific source in a given direction.
Candela comes from the word candle and in general terms a candle would give off approximately 1cd.
Lumen is an SI derived unit used to measure luminous flux. This is the visible light incident on the human eye, or how bright an object looks.
The calculation to convert Candelas to Lumens is a follows.
Candelas = Lumens /(2π(1-cos(degrees/2)))
Because mcd or Millicandela is also a popular rating you will need to know that 1cd(candela) = 1000mcd(Millicandela)