This statement is styled in line
Color is a property. Properties are like the differing attributes of the text. Font-family is another example of a property. Blue is a value. Values are the ways in which the attributes/properties can present themselves. Helvetica and Georgia are values of the font-family property. Inline style rules only apply to whatever is in between their brackets. Not all paragraph tags will change. Using a style tag in the head (internal method) will change the entire document. The h1 header is styled in this manner. I used the h1 selector in the header to denote only to change anything with an h1 tag, rather than the universal selector of *. Selectors are an instruction to alter only the values of properties included in the selected tag. Inline styles override internal header style rules.
External CSS uses a link tag in the head to connect the two documents. rel is an attribute that tells the html that the link is a stylesheet. href is the path for the actual CSS file that we are signaling to use. Here is a link to the webpage with the external CSS file controlling it.
There are two meta-types of measurements in CSS, absolute and relative. In the absolute types, there are px, in, cm, and pt. These directly translate to pixels, inches, centimeters, and points. These are the most common. Their strength is that they denote real world measurements, which can be useful if the webpage needs to be physicalized for some reason. Otherwise, their downside is that they are inflexible. With the wide array of display sizes, especially with mobile, webpages can quickly become ugly and dysfunctional. The most used relative measurements in CSS are: em, ex, lh, and vh. em is a measure relative to font size. ex is relative to the x height of the font. lh is the relative line height. vh is a percent of the vertical height of the viewport. vh can be especially useful on mobile applications. (these descriptions were taken mostly from: https://www.tutorialspoint.com/css/css_measurement_units.htm)