A long bond is one with a long maturity, say 15 to 30 years (there are even some 100-year bonds floating around, and recently a 1,000-year issue was reported). But the long bond, in the language of Wall Street, is the 30-year Treasury Bond.
In general, long bonds offer a chance to lock in an attractive yield, and longer term bonds usually pay a higher rate than shorter term issues. But intermediate term bonds offer most of the advantages with less interest rate risk. That's one reason long bonds are especially suitable to investors who choose to speculate on the direction of interest rates; long-term bonds fluctuate the most for any unit of change in rates.