The fastest way to make a design look unprofessional is to mess up your typography.
To help you out, I'm going over basic typography terms, and some rules of thumb for their use. This post is intended to be a starting post for those who don't know anything about typography.
Types of fonts
Serif fonts have serifs. Serifs are the little feet on your letters. (Illustrated above.)
San means without, so san serif fonts are those fonts that don't have little feet.
Script font looks hand-drawn, often in a style that mimics cursive.
Leading is the space between lines of text.
Tracking is the overall space between letters. Typography that has a lot of tracking has a lot of space between letter - typography with low tracking is tight and squeezed together.
Kerning is the space between two specific letters.
For Fun: Typeface vs. Font
Most people (including myself) use typeface and font interchangeably, but if you want to get technical, they are, in fact, two different things.
Typeface is the specific design of the letters.
Font is the computer program or cast metal pieces that allow you to use that particular typeface.
Typography Rules of Thumb
Design is subjective and takes a certain level of intuition to do well, but that doesn't mean there aren't any basic rules for creating something beautiful.
Use 1-3 Typefaces
Using more than three typefaces will make your design look cluttered and unorganized - it's also incredibly difficult to make more than three fonts works together cohesively.
Give Your Type Plenty of Room
Typography is pointless unless you're able to read it. For increased readability, make sure to give your text a lot of breathing room.
Font Families are Your Friend
A typeface with several different weights and styles is called a font family. Font families make it really easy to introduce variety in your typography without fear of clashing fonts.
When it comes to visual branding, if you want to be memorable, you have to be consistent. This includes your font usage. When creating your brand guidelines, pick one to three fonts or font families, and stick to them for increased impact in your branded materials.
It may seem like a small problem, but in the digital age, when consumers have shorter attention spans, the ability to capture attention with your graphics is more important than ever.