Cycling shorts have a pad built in to to help delay "Saddle Butt Syndrome".
A good pair of padded cycling shorts will let you stay in the saddle all day if you want to.
For those that might feel self-conscious in clingy Lycra clothing, baggy cycling shorts are available in more conventional designs but still with the added comfort of padding.

Cycling Gloves or "track mitts" stop your hands getting sore and offer protection if you fall off. They have padded palms to soak up vibration from the handlebars and some have a gel-pad built in to the palm for extra comfort cycling over long distances Winter cycling gloves need to give a bit more protection. Get cuffs that can fit inside the sleeves of your jacket to keep draughts and rain out. Breathable is best as a build up of sweat can chill and make your hands colder than ever.

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Cycling jerseys usually have their pockets in the back, a zip for ventilation and are often made out of technical fabrics to wick away moisture and prevent you overheating.
My summertime favourites have full length zips so that on very hot days I can open them right up - aaah...lovely breeze!
In autumn and wintertime, long sleeves can be rolled up or down as neccessary while you cycle.

Cycle shoes have stiff soles to transfer more of your cycling effort to the pedals. They are often very well ventilated to keep your feet cool as you cycle. Racing shoes can be difficult to walk in so if your kind of cycling involves time off the bike (sight-seeing etc) then you would probably be better off with the recreational type of cycling footwear which usually look more like trainers. Most cycle shoes nowadays allow you to fit clips to the sole to fasten your shoe to the pedal.

Cycling Jackets are best made of a breathable fabric and should have some means of adjusting their ventilation. Cycling jackets are usually longer at the back and at the wrist to keep those areas protected in the cycling position.
Different cycle clothing manufacturers design different way of ventilating their garments. My favourite cycling jacket (Endura) has zips at the armpits (pitzips!) that help make it comfortable to wear all day in all conditions.