from dusk til dawn (this year its from around 4:15AM-9:15PM):
-no food
-no drinking (includes water)
-no chewing gum/candy/ etc.
-no inhalation of smoke
-nothing that excites taste buds to be placed in the mouth purposely
-no injections/blood withdrawal from body/nasal sprays/medication/halls/antiseptics etc.
if you are sick, old, unable to fast, you are not obligated...but "being hungry" and "omg i am going to faint" are not excuses to break your fast...at the end of the day someone who does break their fast will know deep down inside whether or not it was legitimate or not, its no one elses business nor does anyone else care...
the point is that you go back to basic human existence, back to reality, the reminder that half of the world (probably more) live on a constant fast because they have no means or methods to secure food or water...
throughout whole ramadan:
-observe all prayers (should be all year round, but emphasized even more during ramadan)
-no swearing, bad mouthing, back biting, gossiping etc.
-attempt to spend every moment of it as a model human being, almost to the extent where your main goal it to always be there for anyone...show kindness, compassion, etc.
-give charity, donate food to shelters, homeless people, give food to families who cannot afford to eat
-refrain from sex (which should only be with your Wife anyways the rest of the 11 months out of the year

)