Whats the difference between a first generation antihistamine like chlorpheniramine and second generation antihistamines like allegra (fexofenadine)?

1st and second: First generation antihistamines are sedating and short acting and have usually other properties such as anticholinergic or drying properties. Second generation antihistamines are long aciting, usually dosed once a day, and are less or non sedating and selective antihistamines with less active properties on other receptors.

Sedation: The second generation antihistamines are less likely to cause sedation.