Pretty much sums it up.. Guilds (MG anyway) scrim.. the difference lies in several factors. Guilds mean to me a group that is more interested in helping than competing.
Helping its members get better, and helping the community. Clans are usually groups of people who group together to compete. If MG competes its more along the lines of helping the members gain competition experience, but it isn't the overall goal and happens when the members want it to. Also it has a lot to do with how we are run. Our members have a huge say in the direction and there is no solid leader imposing their will. Equal input for members, and driven by members. Those are the primary differences guild has come to mean in respects to MG.
Also guild can include the aspect that we are more free form allowing our members to join clans by not placing any restrictions on them that would inhibit it, where with a clan being in more than one clan can cause conflicts.