No one has been able to make me understand trigonometry better than engines. I mean parallel lines, intersecting lines, angles between them were some things I never thought would appeal to me unless I got hooked to something that is letting me pay my bills - motorcycles.
In a nutshell, we can call V engines as what we see in a Harley Davidson as the second thing after the logo. Inlines are hard to figure out unless you bend your back a little and figure out how many exhausts are jetting out from an engine. A V engine has the alignment of its pistons in the shape of what lies between U and W in the series of English alphabets - V. Inline or parallel engines have pistons placed literally in parallel positions to each other.
Pistons in a V setup, share the same crank or crankshaft using connecting rods. These engines also consume less space than the same capacity engine in a parallel format. V engines with equal number of cylinders are usually half in size. I mean, do you remember any car that was an inline 12? Neither can I. Anyways, the angle between the axes might differ from motorcycle to motorcycle depending on the performance it is expected to deliver. A V engine also tends to heat up more quickly which makes it deliver a little more fuel efficiency, but also makes it painful in summer heat. The placement of the pistons also make a V engine to have too much of vibrations that gives it a rattling feeling. A V engine also produces more torque at lower rpms because of the power stroke coming from two sides of the crankshaft.
An inline engine is more balanced than its V counterpart because of the equal weight distribution. It also is more easy to play around with as it has just one cylinder head usually and one exhaust manifold. But, everything said and done, you cannot rule out the size benefit V engines have. That is the same reason that has allowed motorcycle manufacturers across the world to have V-twin engines that displace 1,800cc but look as compact as an inline-four 1,000cc engine. But, the power an line-four can provide can be as much or more than a V setup with a much smoother power delivery.
Another benefit on an inline engine is that it can easily accommodate odd number of pistons which is something a V setup doesn't allow to happen. Odd number of cylinders in a V configuration is almost impossible to make smooth as the uneven number of cylinders on both the ends will result in vibrations which might need some kind of balancer to negotiate and the power delivery is expected to be very uneven as well.
Having said that, I would like to conclude this piece by saying that both the engine setups have their own pros and cons (like always), and there is no clear winner here again. Personally, I like motorcycles with V engines (Ducatis, Aprilia RSV4, Yamaha V-Max etc.) as they have a character, a soul in them, and that that soul is not much user-friendly. You can't expect them to respond as nicely as their Japanese counterparts (Honda Fireblade, Yamaha R1, Suzuki GSX-R 1000) which are known for their refinement and smooth power delivery. Even the European motorcycles with inline engines have been doing wonders from sometime and have been created a niche for themselves (BMW S1000RR, MV Agusta F4).