Implemented via DNS, for now... You can see the court orders about sites from TIB's (The Presidency of Telecommunication) query site. http://eekg.tib.gov.tr/
It's a system where every ISP with a license has to abide. They download a list of domains that needs to be forwarded to a server operated by TIB to explain the reason of access blocking. They add these to their recursive DNS servers so traffic goes there. People using another DNS server other than their ISP's (e.g. Google Public DNS, OpenDNS, etc.) are currently able to access.
Highly recommended that people, regardless of location, use encrypted DNS like OpenDNS to prevent monitoring, malicious in-flight modifications and DNS-based censorship.
Right now Turkish ISP's block twitter through their DNS. People using OpenDNS or Google DNS, or any other DNS service other than Turkish ISP's can access Twitter.
However it's expected to see Twitter's IP's blocked anytime soon. In Turkey some websites, especially porn websites are also blocked by IP.
