Are non-coding RNAs introns?
While Ankur's answer is correct, it must be noted that not all non-coding RNAs are introns. 

An intron must be excised from an mRNA, which therefore means that any non-coding RNA that is not part of an mRNA cannot be an intron. 

For example, rRNA and tRNA are all examples of non-coding RNAs that are not introns, since they are not part of mRNA. miRNA may in some instances be produced from introns, but when transcribed separately, they are not introns. 

mRNA translation depends on many factors that bind to processed mRNA (a very long process). Other RNA do not undergo this process, and therefore are not reconised by ribosomes and are not translated.