Quad roller skates (what you think of when picturing traditional roller skates) were designed with two pairs of wheels on each skate that give you great stability when skating and at rest, while inline skates, or roller blades, feature one row of wheels down the middle of each skate and are therefore a little trickier to get your balance when wearing. While the spread of the wheels on quad skates offer increased stability, they also have more of a tendency to roll over even the tiniest obstacles that block the way and cause you to lose your balance, such as skating over a rock or crack in the path, but inline skates utilize wheels with thinner profiles, making it easier to dodge small obstacles or allow for small rocks to be pushed away rather than acting as a block. That's why inline skates are recommended for outdoor use and quad skates for use on broad, smooth surfaces like roller rinks.
Both skates come in a variety of boot styles, including high top and low, sneaker-style boots; however, you will most often find that quad skates come with a low cut boot that doesn't typically offer much support around your ankles, and inline skates have a high molded plastic boot that offers excellent support. This makes inline skates better for activities like speed skating and roller hockey, while quad skates really shine when used for figure skating or roller derby, both of which demand versatility and mobility that quad skates offer.
Generally, quad and inline skates use the same step-like motions to propel yourself forward, and either skate will take some time getting used to. A major difference that could affect your decision though is how you brake while skating. Quad skates have their brakes on the toes in the front, while inline skates have them on the heels in the back. Both are equally effective, it just depends on which way you feel most comfortable.