The first thing you should know is that Key West proper is divided into two specific areas: OLD TOWN and NEW TOWN.

OLD TOWN:  Most Key West maps and even Fodors will say that Old Town is located west of White Street.  Of course this is the easy explanation.  NOTE:  I say "west" for the ease of the traveler but don't be surprised if you ask a local and he says "south."  We (locals) tend to say "south"  instead of "west."  We use south to describe a direction heading away from Miami and north to describe a heading toward Miami.   You will find that most of the homes and structures between White Street and Bertha-First-Palm (one road with three different names) are considered "Old Town or historic."  Old Town is where you find historic architecture but the roads are tight and traffic moves slowly.  Although Old Town is considered the Historic District, there are many structures that are not historic.  Within Old Town, there  are a couple of sub-locations.  One is called " Solares Hill" and it is the highest natural area of the city.  The "hill" is only noticiable if you know how to see it as you near it.  How to find it:  Get on Simonton at Truman traveling south (ha - west - well toward sunset) and you will come up to the intersection of Simonton and Olivia - look at the streets to your right as you pass, you will notice a slight uphill incline - this is the beginning of Solares Hill.  Most locals will laugh and tell you the highest point on the island is the Garrison Bight Bridge on  Palm Avenue.  Most attractions are located in Old Town.  Another area within Old Town is Bahama Village.  This area is located west of Whitehead Street and has a lot of culture.  The Blue Heaven Restaurant is in this area.  Thirdly is Duval Street which is an area in itself.  Duval is divided by the locals into "upper Duval" and "lower Duval" with the La Concha Hotel is the vague half-way point ( remember what I said about local's and their use of the direction "south?"  It applies to Duval Street also - upper Duval is actually the southern half while lower Duval is the northern half - if you use a compass.  Locals use the words "north and south" as they apply to distance from Miami.  Hence, lower Duval is the farthest point from Miami and the buildings have lower street addresses.  The Pier House Hotel is the lowest at 0 Duval Street ).

NEW TOWN:  Again New Town is everything else that is not classified as "Old Town."  You will find shopping centers, grocery stores, a movie theatre, home improvement stores, etc.  Most houses built in new town are typical Florida style CBS homes.  Tourist attractions and favorites in New Town:  Smather's Beach with its 2.5 mile sidewalk, Charterboat Row, and the East Martello Museum. 

STOCK ISLAND:  This is technically a part of the City of Key West but no locals will call it Key West.  It is a separate island connected to Key West by the Cow Key Channel Bridge.  Stock Island was named so because in the early days, it is where livestock was kept.  Attractions on Stock Island:  Hurricane Joe's Bar and Restaurant and the kayaking and fishing trips that run from the marina; Hogfish Bar and Restaurant;  the Rusty Anchor Restaurant and on the other side of US 1 you will find the public Golf Course, Key West's very small version of a Botanical Garden and the Tennessee Williams Fine Arts Center.