i-VTEC changes HOW MUCH the intake and exhaust valves can open. VVT changes WHEN the intake and exhaust ports open.
It's a lot of information to put in an answer. The short version is that by electronically changing exactly when the intake and exhaust valves open and close, you can increase the volumetric efficiency of the engine across a large RPM range. You can also do fancy things like overlapping the exhaust and intake valves so that the exhaust helps "suck" more air and fuel into the combustion chamber, thereby making more power than would normally be available.
Cars without VVT will have a much narrower power curve because they can only work optimally within a set portion of the RPM range, but VVT allows you to enlarge that range quite a bit.
As for I-VTEC, it is basically a way to modify the camshaft profile so that it lifts the valves higher than normal, allowing more air into and out of the engine per stroke. You basically have an economy-focused cam profile when driving slow, and a high-lift performance profile when going fast.
You could also call VVT "dynamic cam timing" and VTEC "dynamic cam profiling".