Tempo: music should be played slowly. The Italian word for 'slow'.

What's the difference between lento and largo?

Generally speaking, largo is used for music which is majestic and stately, whereas lento is a general term for slow pieces.

Let's look at an example: Chopin's Nocturne in C-sharp minor (see video below). The melody is played slowly but it does not have the stately feel of largo. It does, however, create a more intimate ambience:

Another example is Satie's Gnossienne No. 4. Although the legato quavers in the left hand make the piece feel quicker, if you listen to the melody it is played slowly, almost in time with your breath. Have a look at the video below: