"aburriendo" o "aburrido"
Aburrido seems to be used to express the condition of boring, but it is the past participle of aburrir. Why isn't the Gerund, aburriendo, used?
4 Answers
"Aburrido" is used either as a part of a verb phrase (just like "aburriendo") or as an adjective. "Aburriendo" is never an adjective. For example you could say "Este libro es aburrido" (This book is boring) or "El chico está aburrido" (the boy is bored). But it wouldn't be at all correct to say "Este libro es aburriendo" and "El chico está aburriendo" would be correct technically but it would mean "The boy is being boring right now." In English we use gerunds as nouns and parts of verb phrases but in Spanish they are only used in verb phrases and are never nouns. For example in English we'd say "I like running" but in Spanish "running" is not the gerund "corriendo" but the infinitive "correr". So, it's "Me gusta correr" and not "Me gusta corriendo." The same goes for "aburrido" and "aburriendo"--they're not interchangeable. I hope this helps! 
The root of this confusion is that the English gerund and the Spanish gerundio are not really the same at all. You might say the words are "false friends", in that they sound very similar, but mean different things. The English gerund, as Piedra has said, is a verb form that is used as a noun. Example "Skating is a good sport". The Spanish equivalent is not "Patinando es un buen deporte". In this case, you use the infinitive, "Patinar es un buen deporte".
The Spanish gerundio is more equivalent to the English present participle. The confusing thing is that in English, the gerund and present participle are the same word, only differing in useage.
Estoy aburrido = Me estoy aburriendo. Absolutely the same, maybe the second is more "demanding" the listener help!.
I believe 'aburrido' is used to mean 'bored', so it's much the same as in English.
Note, it's important, if you're bored, to say estoy aburrido.
Soy aburrido is something a bit different. 
