The ethereum.org site says one can create a "tradeable token with a fixed supply" on top of Ethereum. What does this exactly mean?
What if I wanted to create a token with variable supply - I mean maybe a pre-set supply scheme (like Bitcoin) or a supply that will be agreed on by the clients/users (via some kind of consensus)?
It means the total supply is fixed. You could write smart contracts to predetermine how the token will be distributed (releasing more tokens over a set time period, etc.) however the total supply will not change - only the "circulating supply" will. Ethereum, Nxt, Couterparty all allow users to create their own tokens within the Ethereum, Nxt, Counterparty protocols, respectively.
Variable supply is fine - I think they used the word "fixed" to more easily convey what issuing your own token is all about.
As it is your own token you have the freedom to do whatever you want with its supply - e.g. you may set totalSupply at 1,000,000 in the beginning and add a function changeSupply to increase or decrease totalSupply at will. Same for issuance.
I do want to point out the ERC20 standard, which you might want to consider if you want your token to enjoy some broader adoption down the line. Note it comes with a totalSupply function, but all it does is return a value.
