100000 is the number of competing pages for the keyword. To find out this number put the keyword like so "keyword" in double quotes (this enables phrase search) Then in the top right of google it tells you how many pages there are. This number should be less than 100000.
To clarify on Imkanes info. Local , is the previous months searches for the location selected at the top of the tool. (you location can be changed) It is important to understand that the previous months results may not occur every month. For example the keyword "xmas paper" will have a very high local search volume in december but almost none the rest of the year. Personally I do not like the local search term as it can be very mis leading. The Global Search volume, is the average volume from all countries. This when combined with the trends info gives you a far more realistic snap shot of your search volume. If you want more info on local searches I strongly suggest you use google insights or trends.
It is some what difficult to find two word key-word based domain name. Though many do find such domains but they have to make hundred of searches to find the real perl. Three word domains are still acceptable as long as they comprised of eyecatcher keywords.
I agree, but don't underestimate the power of a prime keyword url. i.e. on with no dashes and is .com, .org, .net These URLS can really help with hitting top ten easily