Directional mics you have to speak directly into the front of the mic, or put the front of the mic directly on the speaker of the instrument you are mic'ing.. Directional mics are best used for instruments, because they don't pick up as much noise from the sides.. Example is a Shure SM57 mic, which is the shape of a cigar, sort of.. just a pipe with mesh on one end for the mic part.
Unidirectional mics can pick up signal from all directions and are used more for vocals.. an example is a Shure SM58 mic which looks like a pipe with a ball on the end made of mesh metal..
Of course there is no absolute science in music, so you see people singing into 57's and mic'ing instruments with 58's.. When I play, I use two 57's on my amps and I mic two different speakers.. vocally I use a 58 for whatever little vox I do...
Here's a pic of both..
Friendship is like peeing your pants.. Everybody can see it, but only you can feel the warmth..