A section of the Codex Alexandrinus containing Luke 12:54–13:4.
The comma (,) and full-stop (.) are used as in English. The colon or semicolon is a point above the line (·). The Greek question mark ; looks like the English semicolon.
Inverted commas are often used to denote speech. Capital letters are used at the beginning of paragraphs, sentences (depending on publisher), proper names, and the beginning of quotations.
In actual Greek texts from the era when Koine Greek was used as a day-to-day language, Greek was usually written with no punctuation. The words ran together completely, with no spacing or markup. Accents, breathing marks, spaces, and other punctuation are added at a much later time, making texts easier to read.
Many of the earliest partial manuscripts of the New Testament do have punctuation. Due to the high cost of the factors of input (ink, paper), punctuation was quickly excluded, to be re-included later by textual scholars (not unlike the adding of vowel-markers to the Biblical Hebrew and Aramaic texts by the Masoretes).