tagname: selects elements with the specified HTML tag name
tagname1, tagname2: selects elements with either specified HTML tag name
tagname1 tagname2: selects elements with tagname2 which are descendants of an element with tagname1
.className: selects elements with the specified class name
#id: selects the element with the specified id
tagname.className: selects elements with the specified tag name and class name
Searching for elements based on CSS selectors
document.querySelector(cssSelector): returns the first element in a depth first tree traversal that matches the specified CSS3 selector
element.querySelector(cssSelector): returns the first element in a depth first subtree traversal starting with element that matches the specified CSS3 selector
document.querySelectorAll(cssSelector): returns a NodeList of all elements within the tree that match the specified CSS3 selector
element.querySelectorAll(cssSelector): returns a NodeList of all elements descended from element that match the specified CSS3 selector
jQuery uses the $() function which serves many purposes and returns a jQuery object which is a collection of the elements found. jQuery then provides a multitude of methods that can act upon the elements of that collection.
CSS specificity
See the following resources for details on CSS specificity.