That's a tricky question to answer! Love has long been the subject of philosophical, religious, and biological discussion. We can't see or touch love as an independent entity, so how do we study it? Is love a feeling, an action, or a hormonal process? Let's consider love from each of these positions.
First, what does love feel like? Love can be made up of or co-exist with many other feelings like affection, parental care, friendship, desire, happiness, even worry. Loving someone or something involves caring about that person's (or thing's) well-being.
Which leads us to the second part-- the action of love. How is love enacted? Many actions can contribute to the development of love, its growth, and maintenance. This is perhaps the most difficult part of answering your question, because the way we act out love are very personal. To speak quite generally, love may be acted out through physical affection like a hug. But not all acts of love require physical contact. Wishing someone a good morning an be an act of love. Giving a gift can be an act of love. Cooking a meal for someone is a very primal act of love because it demonstrates care for a person's needs in life. Some of the ways we act out love are long-term, like marriage.
So, what's at the root of these feelings and actions? Oxytocin is often called the love hormone because it is a chemical released in the body which triggers the sort of "warm, fuzzy" feelings I described above. Our first encounter with oxytocin in life is when we our born, when we receive it through our umbilical cords and through breast-milk. This hormone helps both mother and baby to feel good and form an emotional bond, recognizing each other as a trigger for this feel-good hormone. Later in life, oxytocin is released during positive social interactions (like a hug or sharing a joke), intercourse, and even when we interact with our pets. When we grow to love someone, we prime ourselves to have a strong hormonal response to that person.
In a way, love is a cyclical process of a feel-good hormonal response to certain actions or interactions, which prompts us to seek further positive actions and interactions!