What starts as acquaintance slowly turns into friendship without one deliberately thinking about the relationship.
If we think along the lines of a continuum, acquaintance is at the left extreme followed by friendship while relationship lies at the far right of the continuum.
In most culture, there are some basic behaviors that are indicative of mutual friendship such as holding hands, kissing each other on cheeks, exchanging bracelets and friendship bands etc.
In friendship, feelings and emotions play a very important role, and friendship could be asexual or sexual.
It is when one feels comfortable in the company of another person and likes to spend time with him that friendship is said to have been formed.