Selectors

// Basic selectors
$("#header");              // by id
$(".active");              // by class
$("div");                  // by tag
$("*");                    // all elements

// Combinators & attributes
$("ul li:first-child");    // first list item
$("a[href^='https']");     // links starting with https
$("input[type='checkbox']:checked");

// Form & content filters
$(":button");              // all buttons
$("div:contains('Hello')");
$("tr:odd");               // odd rows (zero-indexed)