Double-tap any text you are editing in order to select a word. That also allows you to drag the endpoints. Tapping with two fingers selects the whole paragraph.
In short: Tapping places the cursor (and brings up the keyboard if it isn’t already up), double-tapping selects a word, tapping with two fingers selects a paragraph. I couldn’t find any other gestures.
If you you double-tap and hold, you can immediately change the selection, but you are anchored to the word you selected (i.e. you can add characters to the left or the right but not both). Tapping and holding with two fingers allows you to change both endpoints with your two fingers.
Web views (and most other views of text you cannot edit) don’t work like that. In those, you tap and hold to select text. It’s a bit inconsistent, though: in iBooks (for example), tapping with two fingers will select whole paragraphs.