Grooming is the process by which a sexual abuser or potential abuser earns the trust of not only the child, but the people around the child in order that they might have access to alone time with the child and an opportunity to be sexual with the child.
Sometimes, grooming is confused with sexual abuse itself, and sometimes grooming is discussed as an intentional process. Sometimes, grooming tactics are sexually abusive, such as showing pornography to a child, and very rarely, abusers are intentional and knowledgeable about grooming children for abuse, such as recidivist sexual addicts, those with antisocial personality disorder, and some preferential offenders.
However, much of sexual abuse is situational, or arises out of complex motivations that have nothing to do with a desire to be sexual with a child or cause them pain. It is also possible that the motivations behind behaviors that are considered signs of grooming, like gift-giving, are completely innocent and not related to sexual abuse. It is also possible to be engaged in the process of grooming without being aware that the situation may lead to the sexual abuse of a child. Thus, grooming must be treated with an ounce of caution.
Most of the time grooming is happening, you will not be very aware of it, as it is normal and expected behavior from a trusted friend or family member. Some behaviors found in lists of grooming behaviors are more intentional, some are less common, and some seem so innocent you would hardly expect them. Sometimes, things that look like grooming and "make the list" are adults concerned for a child in their lives, and sometimes, things that do not look like grooming and are not on a list are grooming and need more attention.
Regardless, these lists can be helpful in the right circumstances. One thorough example of a well-balanced list can be found here. What makes something grooming and something else perfectly innocent depends entirely on the internal motivation of the person, which means that looking at grooming signs can be tricky and can paint only a partial picture of the actual situation.
Grooming is very similar to the warning behaviors, but they are more specific and not always as observable: Where grooming depends on motivation, which is unseen, the warning behaviors are things you can see, which is why this site emphasizes warning behaviors. The presence of apparent grooming behavior and these warning signs may be cause for you to have a conversation with the person, or reporting your suspicions to the police. If they are someone you care about, you may want to try to help them: This resource can help give you starting ideas for how to do that.