A hen who is actively sitting on a nest, trying to hatch eggs, is called broody.  Any breed can go broody but some are more prone to than others.  We work solely with landrace breeds where broodiness is a desired trait.  

When a hen goes broody, her body temperature will increase, she may pluck feathers from her breast area for the nest, and she will probably eat and drink less food.  It is important that you keep an eye on your broodies so that they don't lose too much weight.  She will still get up to eat and drink, but chances are you will not see her.  She will likely become aggressive, puffing up her feathers and clucking aggressively or growling (yes a hen can growl) and peck trying to protect her nest.  Be wary around her nest just in case.  

In the photo, The Karens (wooly Hedemora hens) are sharing a nest (kitty litter bucket.  Hey, it works.  Wolly Hedemora cannot fly up into a nest).  It is not uncommon for multiple hens to share a nest area.  At one point, there were 3 in that bucket.  Since then, Karen 1 moved to a nest of her own beside the bucket.  We also have a Swedish Flower Hen sitting on a nest but her nest is up high.  We will have to try to move her nest to the ground and hope she doesn't then change her mind about sitting.  

It takes 21 days (3 weeks) for chicken eggs to incubate.  If you want your hen to hatch her clutch, then be sure to mark your calendar so you know when to expect chicks.  Have chick food and water available at ground level so that they can eat.  Give the eggs a couple of days to hatch unless mama has left the nest.  Be careful with any unhatched eggs so that you don't end up with a stink bomb.  

Any hen can go broody, whether you have a rooster or not.  If you do not have a rooster but want your hen to hatch some chicks, you can find some fertilized hatching eggs for her to sit on.  Some people have had luck setting day old chicks under a hen but we have not tried that method.  Keep an eye on them if you go that route so that mama does not kill them.  (Yes that can happen)

If you do not want chicks, you can break a hen from broodiness.  Different ways for that are: 1.  Keep her in a pen with a wire floor for a few days.  2.  Collect eggs regularly and don't allow her to sit in the nest box.  3.  Set an ice pack or the equivalent under her to lower her body temperature.  

Whichever way you decide to go, be ready for henhouse drama for a bit of time.  Be ready to get talked back to and pecked.