Some more specs:

• 1 oz., enough for thousands of joints. Check out the solder section of our soldering guide for lots more information, including the testing that backs our estimate that 1 oz will last for thousands of joints.
• #66 core (3.3% flux by weight)
• #285 RMA flux (RMA = Rosin Mildly Activated), flux is classified as ROL0 under IPC J-STD-004. What does all this mean? This solder will readily wick to metal surfaces, and it's highly unlikely that cleaning is necessary. See the flux section of our soldering guide for more information about what all the parameters mean.
• solder data sheet

We recommend this solder over:

• water-soluble solder: This contains extremely aggressive flux that is unnecessary for all but the most difficult to solder or heavily oxidized components. Its residues are corrosive and conductive and must be cleaned off with warm water. The mildly activated flux in the Kester solder on this page shouldn't require cleaning unless you want to be absolutely sure your project will last for 5 years in a hot environment. Water soluble flux will also erode tips faster than RMA flux.
• no-clean solder: No-clean solder has very little activation, and is used in critical applications. It won't perform as well on parts that have some oxidization, so we recommend the mildly activated flux over this (which also doesn't require cleaning in most situations). No-clean is not a very well defined term, and doesn't necessarily mean the residues should not be cleaned. It simply means that the flux is not very active, and in most situations the residues don't require cleaning. Many manufacturers clean no-clean residues despite the name.

What about 60/40 solder vs. 63/37? A while ago, the electronics industry switched to 63/37 (and now they're switching to lead free) because it improved yields in mass soldering of surface mount components. 63/37 has slightly more tin, which should increase its cost slightly. It also is a eutectic mixture which means it freezes at one temperature like water, whereas 60/40 goes through a pasty range while freezing. Does this matter? In our experience hand soldering, not noticeably. A glass of water freezes at the same temperature, but freezes from the edge towards the center, not all instantaneously. So while there is a larger time that a 60/40 joint could get disturbed while freezing, the increased time isn't necessarily significant, and a 63/37 joint doesn't necessarily freeze instantly. We recommend getting a good brand, and whatever is cheaper. New to surface mount soldering? Check out our introduction video: Surface Mount Soldering 101.