When looking for an apartment, I often see 契約期間:2年 written. What does it mean? I have to make a contract for 2 years and then it can be renewed or extended with some extra renewal cost? Can I rent just for 1 year?
As a sentence of Japanese, it just means the contract is valid for two years.
Practically, first you have to check whether it's 普通借家契約 or 定期借家契約. The latter is uncommon but it means you cannot extend the contract and have to leave your apartment after the contract expires (3 years, for example). See: 「普通借家契約」と「定期借家契約」
Unless explicitly mentioned as 定期, it's 普通借家契約, which means you can basically rent the apartment as long as you want. Still, you need to renew your contract usually every two years. You may be charged additional 1 or 2 months of rent money under the name of 契約更新料 ("contract renewal fee") unless otherwise mentioned as 更新料無料. In any case, you can leave your apartment before two years pass without penalty provided you notify the owner one month prior to the leaving date.
But there may be exceptions on a case-by-case basis, so you need to understand the contract. You can have a detailed explanation before you make a contract.
In japan You usually have a minimum contract of 1 or 2 years. If you leave earlier you will need to pay a fee worth 1-3 month rent. There is probably no problem in staying longer. If you need to renew every time is a question better asked to the owner. But it's probably not an immediate problem.
Where I stayed in my first apartment in Japan, no renewal was required, but if I didn't stay at least 1 year(I didn't) I had to pay 3 month rent(or so said contract I signed, but the owner said I didn't have to pay when I left. lucky me).