Directly or subtly expressing your physical or romantic interest in someone - that's it.
There are no 'rules' to flirting.
Even if you approach a woman but are so nervous you stutter when you speak and sweat, that's still 'flirting' as you must find her so attractive, you're having trouble talking to her - you're expressing your interest in a non-direct way. But, even just approaching to talk to her in the first place, is a direct way of showing you're interested in them, especially in a situation such as a bar or other social place where flirting is commonplace.
Many people though, flirt 'just for fun' and have no romantic intentions of any kind at all - avoid at all costs if you want a serious LTR, or even a one-night stand.
"I was just making comment or getting my point across."
What kind of 'comments'? If you're saying to female friends and complimenting them on their looks or possible aspects of their personality, unless it's well and truly, 100%, completely and totally established you are both friends and nothing more, they may think you are interested in them romantically or physically.