It's a coin toss to decide whether mechanical engineering is more or less prevalent than civil engineering. They have some definite overlap - both are dealing with concepts like static and dynamic forces (standing on a bridge versus walking on a bridge), bending moments (what happens if I put this much pressure on a beam which is so-many metres long, with a support at the other end? It bends...), and materials. I guess the easiest distinction is that mechanical engineers are usually more interested in things that move*. It's also been said that civil engineers build targets and mechanical engineers build weapons to break them.
Mechanical engineering shows up all over the place: cars, trucks, trains, aeroplanes, ships, bicycles, escalators, elevators, refrigerators, mining, just to name a handful.
Materials engineering is usually considered a subset of mechanical engineering, and is particularly interesting. Materials engineers do a couple of things. One, they take existing materials and work out what its characteristics are. That is, if I take a rod of material X, will it rust? Will it pass electricity? Will it pass light? How dense is it? Can I crush it? Will it bend or will it break immediately? When it breaks, does it send thousands of deadly projectiles across the workshop? Does it burn/explode/other cool but dangerous things?
Another thing that materials engineers do is to come up with improved compounds for existing materials, or brand new compounds that have certain desirable characteristics. The "composite materials" used in the new Boeing 787 are one example of this.
Like their civil cousins, mechanical engineers do some pretty spectacular things: aircraft carriers, ferraris, or my personal favourite, the Lockheed Martin SR-71 Blackbird.
Also like their civil cousins, mechanical failures can be difficult to hide, like the Hindenburg.
Tomorrow, electrical engineering.
*Not all mechanical engineers are interested in things that move. Where I work, things don't move but our mechanical engineers are very interested in things like how to get heat out, how to make it light but still strong enough to be bolted to an aeroplane, stuff like that.