I'm ignorant about bikes so...In the listing, 26' road/mountain bike, what does the 26 indicate? thx 4 ne info

Is it a measurement of the frame or is it the diameter of the tires? Is it about height?If it is the tires, what are the pros/cons of have a high numbered tire(26, 25,etc)versus a lower numbered (21,20, etc.)If its bout the frame and tire, could you put a 26' tire on a bike listed as 21', whatever that means. Furthermore, 4 adults who want 2 ride in NYC tri-state area city terrain, is it possible 2 have a mountain bike w/ front shox(4 inevitable potholes/jumpin curbs)but with slicker tires rather than the knobby? wud dat b expensive? If a bike's listed as 10 speed/15 speed/21/ and once I saw a listing for 24spd., what is the difference? Does the 10 mean 10 different gears or it can go 10mph, etc?Confused, if it were so then y not make a 35 speed or 62 speed bike, etc.? I know this is a lot(u may email if desired), but I'm thinkin'bout gettin' a kmart/target/used mountain bike w/shox for durability, then having slix put on it for speed, but I'm unsure if this is possible or 2 expensiv.

wow.....
okay.., 26 indicates the wheel size and it's the wheel size you see the most- e.g. all mountainbikes are 26". all road bikes come in 700cc wheel size.
21" is a kids bike wheel size and you can put only 21 size tube and tyre on it.
yes, if you buy a mountainbike and you don't like its knobby tyres, you can change them to whatever you want. it's as expensive as buying a new tube and tyre. although I don't see the point to have a mountainbike and ride with slick/thin tyres- if you wanna have something like a mountainbike but want to use it mainly on the road or in the city, you can buy a hybrid or those specially made city bikes which pretty much resemble a mountainbike. something like this: http://www.rei.com/product/760870 .

bike "speed" is nothing important to you. all you need in the city is not more than 10 gears. sometimes people even buy a fixed gear bikes (http://en.wikipedia.org/wiki/Fixed-gear_... ) cause there's no need for any gears on smooth terrain. 24-speed bikes is a necessity for racing. you don't need that at all in the city. personally I have my 24 "speed" racing steed and another one for the city which is the fixed gear bike.
how it works? 10mph? haha, no. that's why I really don't like the word "speeds" which is a misconception itself. without going too far into it you can learn about bicycle gearing on here: http://en.wikipedia.org/wiki/Bicycle_gea... ...
as I said it's up to you- I, as a racer need 24 gear bike only for the competitions. for amateur mountainbiking there's definitely no need for more than 21. for the city my choice is the fixed gear.

also, having a a suspension fork doesn't mean the bike is durable, actually it's quite opposite. actually the most durable thing you can get is having a cromo (steel) fork. and in my opinion, for the city there's no need for the suspension fork at all. good suspension forks start around 350$. I don't think it's neccessary to spend this money for the ride in the city.

A 26" bike usually refers to Mountain Bikes (MTB) and their wheel size, a 21" is usually a kids bike. Road bikes aren't usually described in this manner(as it is assumed they use 700c wheels - don't ask!). A smaller wheel size will be easier to pedal at the expense of greater top speed.

Yes you can get an MTB with front shocks and put slicker tyres on it if you want to! it would only cost the price of two tyres.

The speed thing you mention is down to gearing and how many combinations of gears you can theoretically use (eg. if you have 8 cogs on the rear cassette and three cogs on the chainset then you have 24 gears)

Most road bikes have two chainrings on the front and 8,9 or 10 cogs on the rear giving you 16,18 or 20 gears respectively although you can have a triple chainset which will give you 24, 27 or 30 gears respectively.

The question you ask is quite involved and I could spend all day discussing it but I hope this gives you some idea.

.