An SSD as a high-performance hard drive, without any rotating parts using memory for storage rather than disk platters like traditional hard disks. Because it has no moving parts it offers "almost instant" (~<1ms seek times as opposed to mechanic disks with ~>8ms) seek times, and offers considerably faster read and write times.
It's without question the single best upgrade component a computer user can add to a computer system. Windows will load considerably faster (mines booted to login screen inside 12 seconds on average and ready on the desktop within 5 seconds of entering password) and applications (installed on the SSD) benefit from faster load times.
Motherboard and CPU are fine for now, they're getting old now but perfectly servicable given the type of usage the computer is used for. I still use a quad-core Q6600 of the same generation as your CPU in my main PC, and although I'm pushing the boundaries in terms of performance with my usage (Virtual machines, dev, office, gaming) it's hanging in there just fine. Your demands are much lower in terms of the resources your computer provides, so you could safely hold off upgrading the motherboard and CPU for now, and concentrate on other areas (SSD, bigger disks, nice case, PSU, optical drives, keyboard/mouse etc) for now and upgrade them next year once Intel's next generation CPUs arrive.
