Turkey Time, abbreviated TRT, is the country's single time zone. It is UTC+3, meaning three hours ahead of Coordinated Universal Time, and it applies to every province without exception.
You will sometimes see it written as GMT+3. For everyday purposes UTC and GMT are the same thing, so both are correct. The IANA database identifier — the one used by phones, servers and this site — is Europe/Istanbul.
Before 2016 Turkey behaved like its European neighbours: UTC+2 in winter, UTC+3 in summer. On 7 September 2016 the government announced that the clocks would not go back that autumn, and that the country would remain on UTC+3 permanently.
Turkey therefore sits on what used to be its summer time all year. This is why sunrise in Istanbul in December is after 08:20, and why the offset between Turkey and Europe is one hour in summer but two in winter.
Turkey shares its offset with Moscow and western Russia, Saudi Arabia, Qatar, Bahrain, Kuwait, Iraq, and East Africa including Kenya, Tanzania, Uganda, Ethiopia and Somalia. All of those keep UTC+3 year-round, so the time in Istanbul, Riyadh, Moscow and Nairobi is identical.
Greece, Bulgaria, Romania, Ukraine, Israel and Egypt also reach UTC+3, but only during their summer months, dropping back to UTC+2 in winter.
GMT+3, permanently, since September 2016. Sources still listing Turkey at GMT+2 for the winter are out of date.
Europe/Istanbul. Older systems may also recognise the deprecated alias Asia/Istanbul.
Only in summer. Greece is UTC+2 in winter and UTC+3 in summer, while Turkey stays at UTC+3, so they match from late March to late October and differ by an hour otherwise.