Many times I hear people using loneliness and solitude as interchangeable terms. There is a fundamental difference to between the two words, and I'm going to clear up this common misconception with a simple explanation.
Loneliness is when one is alone, not by choice, but because of the situation or the people around him. The lonely man strives to be with others when they are not willing to be with him, for which one says that one is suffering loneliness. It's a sentiment that can consume you on the inside to the point of changing you, sometimes fundamentally, and most of the times it's not for the better.
Solitude is glorious. When one is alone by his own choice, to organize his thoughts, clear his head, or any one of the many possibilities, one is said to be enjoying solitude. This state of person and mind is rare to find in an extrovert but very common in an introvert, and is not comprehended by those who constantly seek to be with others. Some need more, some need less, but it is always one's own choice and for one to enjoy.
Now that you understand the difference, you can see that although they are similar, they are also opposites, as one implies suffering and the other implies, if not joy, at least peace and a calm state of mind. Keep in mind though, the solitary man can sometimes have too much of it, in which his solitude will become loneliness, but of course, those who know how to be alone don't suffer loneliness for long.