Microevolution and macroevolution mean slightly different things to the scientific community than to Creationists.
If we start with organisms and work our way up, we branch into populations within the same species and then communities of populations from multiple species.
Microevolution is change in the gene pool of a population over time. So evolution in a population of birds from one species in one location would be microevolution.
Macroevolution is change at the species level or higher. This can be the result of several microevolution changes over a long time scale, or it could be something more dramatic such as a bottlenecking or mass extinction. For instance, if a disaster hits an ecosystem and kills off large amounts of organisms from multiple species, this could result in a change in multiple population gene pools in a very short period of time. Natural selection is just one mechanism by which evolution takes place; any event that changes the gene pool causes evolution.
The Wiki page on macroevolution
talks about whole genome duplication in plants. I don't know much about that subject, but it sounds like an area you could look into for details on larger scale mutations.
The things a lot of Creationists forget, though, is that species is a man-made classification. We usually made the distinction based on mating, and there are quite a few species that fall into the "gray" area when their various populations mate: they might have the potential to cross-mate but they don't, or they may be willing but make nonviable offspring, or they may be willing in lower frequencies between populations than within, etc. Some Creationists get around this by talking about "kinds," but this isn't something recognized by biologists and I've never really seen them attempt to offer a physiological delineation between these "kinds."
So the question to me is, "Where's the barrier?" And I've yet to hear a precise answer on that. Sure, you'll get separate evolutionary paths - other primates and modern humans, or if they're feeling facetious, dogs and cats. But you probably won't hear them say, "There's X factor that prevents one population from ever creating a new species/genus/family/whatevs." And if you do, be sure to let us know so we can discuss it and see its merits.
As for mutations, they refer to changes in DNA during DNA replication. DNA is coded with 4 bases (A, T, C, G) that work to make proteins. For simplicity I'll only talk about the ones actively used in coding to proteins.
When a protein is ready to be coded, part of the DNA unwinds and translates the unwound section onto a piece of RNA. This RNA travels to a processing center called a ribosome, which reads each code 3 letters at a time. Simple math gives us 64 possible 3 letter codes (AAA, AAC, AAG...). Each of these codes is mapped to one of the 20 amino acids (yes, this means there is redundancy in the coding.) The ribosome strings those amino acids together until it receives the STOP code. The completed structure is a protein.
A mutation is a change to a DNA code. A code (AAG) can have a base substituted substituted (AC
G), reversed (GAA), moved to another place, deleted (A*
GC), inserted (AA
A), etc. Some of these mutations are worse than others: a substitution means one amino acid is off, while a deletion or insertion changes everything after it (called a frameshift). Accidentally adding or removing a STOP code will drastically change the length of the protein, and a frameshift will make something completely different.
For proteins to function, they have to fold and bond. As a result, screwing up their amino acids almost always makes the protein nonfunctional. Only rarely does this create a functional protein, and when it does it's usually a neutral change. This is why Creationists argue that mutation can only be deleterious. For us to get anything useful out of mutation, they argue, we'd have to have several lucky beneficial mutations in a row without killing the organism.
At face value that argument seems somewhat reasonable, but it's simply not true. Beneficial mutations may be rare but they do happen, and if they're of significant benefit then they'll spread through the population. And neutral mutations might not seem very special, but multiple neutral changes can have a net benefit. This is especially true because organisms are pragmatists, and will use a tool however they need to for survival. Sure, your limb might not have specifically evolved to climb a tree, but if it's a matter of life and death, you're going up. And if you have a "neutral" mutation granting you stronger nails... well, it might not be so neutral for you after all. And in a few generations, those nails might be claws and your descendents might be tree dwellers.
The other thing to remember is that mutation means a different thing to complex organisms than it does to bacteria. Your body has several safeguards to keep mutations away, while a bacteria does not. So for us, our evolutionary changes tend to occur along traits already in our population rather than new mutations. Kill off all the tall people, and the average height goes down. Kill off all the short people, and the average height goes up. Kill off everyone in the middle, and you're left with two populations: giant and dwarf. Keep the separate long enough, and they'll evolve into two separate species.