★ and ☆: black star and white star
The two classic text stars are ★ BLACK STAR (U+2605) and ☆ WHITE STAR (U+2606). As with hearts and card suits, "black" means filled and "white" means outlined. They have been in Unicode since 1993 and work on practically every phone and computer.
The yellow ⭐︎ is a different character, WHITE MEDIUM STAR (U+2B50), which phones show as an emoji. 🌟 is a glowing star, 🌠 a shooting star and ✨︎ the sparkles emoji. Use ★ when you want a star that matches your text, and ⭐︎ when you want color.
Other kinds of stars
The Dingbats block adds decorated stars: ✩ (stress outlined), ✪ (circled), ✫ and ✬ (open or black center), ✭ and ✮ (outlined black), ✯ (pinwheel) and ✰ (shadowed). The small stars ⭑ ⭒ and the math star operator ⋆ are handy between words.
Stars with a different number of points have their own uses. ✡︎ is the six-pointed Star of David, ⛤ is the pentagram (see the pentagram symbol page), and ✶ ✷ ✹ have six, eight and twelve points. The four-pointed ✦ and ✧ are better known as sparkles, collected on the sparkle symbols page. For reviews with half points, see the half star symbol page.
How to type a star
Windows has no Alt code for ★; Shift + 8 only gives the asterisk. Press Win + . (period) to open the emoji panel for ⭐︎, or in Microsoft Word type 2605 and press Alt + X for ★. On a Mac, press Control + Command + Space and search for "star".
In HTML, ★ gives ★ and ☆ gives ☆. In LaTeX, \star prints the small star operator ⋆, and \bigstar from the amssymb package prints ★. On phones, search "star" in the emoji keyboard, or copy the text stars from this page.