DOMS, Delayed Onset Muscle Soreness happens because when you workout you split the z lines of the sarcomere and they become irritated.
Sarcomere = Small sections of muscle cell joined together to create a muscle connected in a series of strings, between each cell is the z line which connects each cell together.
They body will either rejoin the z lines or grow another sarcomere in it's place
and no, don't work out that part of the body if it's REALLY sore.. a little sore, yeah sure just don't go wild.. but not really sore.
Many people recommend "active recovery" to get blood flowing into the muscle but everyone is different for me it doesn't do shit haha