US airspace is in a nutshell
Class A - Above all, fl180-fl600 IFR only, DME required above FL240
Class B - Busy and Busted, large airport terminal areas, it is busy and you will get busted if you fly in it without a clearance.
Class C - Crowded, surrounds airports that don't fit in the Busy category you need to be talking to someone to fly in it but a clearance is not required (be warned that many US class C airports have more annual movements than London Heathrow)
Class D - Don't fly here unless you are talking to the tower. Surrounds airports that don't get into the crowded category (it can include some very busy places, Long Beach for example)
Class E - Everywhere, that is almost everywhere in the US that is not one of the other classes, you do not need to talk to anyone if you do not want to but if you feel inclined someone will be there to listen and respond. Class E is widespread especially in the lower 48 states. E airspace can go all the way to the ground around non towered fields that have instrument approaches.
Class F does not exist in the USA
Class G - Uncontrolled, only found in places like Alaska and the Southwestern deserts, prairies, and small pockets here and there(below 700 or 1200ft above ground level), usually in remote and unpopulated areas. No services available and no one to talk to.
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In the continental United States you will have to look long and hard to find a place that Radar service is not more than a climb away.
There are some anomalies to the system like the airspace that surrounds Palm Springs. I can never figure out exactly what a TRSA is but I fly it like Class C and it works just fine, it stands for Terminal radar service area.