Gearup2 and others above have summed it up quite well.
There are also quite a number of rules applicable to VFR flight, including different visibilities/distance horizontally and vertically that must be maintained from cloud in different categories of airspace.
The instrument rating, allowing one to fly IFR, is a separate license endorsment, that one learns after learning to fly VFR. The aircraft also has to be appropriately equipped with the correct instrumentation and backup systems. So it's not just a matter of "changing from VFR to IFR", without the appropriate rating and aircraft, and a bit of planning as well.
Unfortunately there are always accidents where VFR rated fliers, for all sorts of reasons, fly into weather conditions that are outside their abilities.
("Get-home-itis" is a common one.) Such accidents are usually fatal, as they tend to involve a loss of control of the aircraft, low level, due to a lack of visual reference. One of the most famous of these was JFK Jnr.'s crash near Martha's Vineyard a few years ago.