Being in love is more of a temporary condition where the senses and emotions are at their peak. People in love experience obsession, fascination, blindness to flaws and faults, and other very intense emotions. This cannot last forever because it is a distracting and exhausting process. This applies to friendships as well as familial and romantic love.
Loving someone is a more permanent condition where a fixed and immutable form of love exists. Love persists even when trust, respect and patience have been worn out. Even affection can go, but love will still be there in some form.