how to type “smart quotes” (U+201C, U+201D)

With a Compose key:

• Compose , " → „
• Compose < " → “
• Compose > " → ”
• Compose < ' → ‘
• Compose > ' → ’

You can enter the last two characters in either order, at least in most locales. There may be locales where it only works in the order Compose < " works or only in the order Compose " <.

If you need to press Shift or AltGr to enter some of these characters, you'll need to hold those modifiers down as usual. You can press and release Compose before the subsequent characters, or you can press Compose and keep it down, as you prefer.

Most keyboards don't have a key marked “Compose”, but you can configure one and your environment may already have one configured. On a PC, a popular choice is the “Menu” key between the right Windows key and the right Ctrl key. The keyboard settings of most desktop environment include an option to make this key or some other key a Compose key.

In Gnome, you would press and hold down Ctrl+Shift, then type u201c.

Of course, that won't work in Gnome Terminal if Ctrl+Shift+c is bound to Copy, in which case type it in GEdit and paste it in, or learn how to enter it in your editor of choice.

Some ‘smart’ [1] quotes without using Compose [2] (and without remembering Unicode codepoints):

AltGr + 9 = ‘

AltGr + 0 = ’

AltGr + [ = «

AltGr + ] = »

AltGr + Shift + [ = “

AltGr + Shift + ] = ”

These work on any desktop, provided you have ISO_Level3_Shift assigned to your AltGr key and are using a keyboard mapping like the US International Keyboard with AltGr.

For Compose combinations, check Gilles' answer (or the XCompose file for your locale). They, in turn, are dependent on your locale and/or the presence/settings of your ~/.XCompose file.

[1] ‘Typographical’ is probably more appropriate. ‘Smart’ refers to the ability of a word processor to automatically use the correct character in a quote pair when you type the ASCII quotes ' or ".

[2] You can, of course, have both Compose and AltGr enabled. I do, and it works a treat.