As you've probably read or heard at some point in your gardening experience, a weed is simply "any plant growing where it's not wanted."
With this definition in mind, a dandelion growing in the middle of an otherwise perfectly manicured lawn could certainly qualify as a weed, unless . . . . . , you just moved into the home and had a particular fondness for dandelions?
If that were the case, the millions of perfectly manicured Kentucky bluegrass, fine-leaf fescue and perennial ryegrass plants in the lawn might be "weeds?"
In other words, which plants are weeds and which aren't can often be a matter of personal preference.
So, depending upon which plants annoy you the most, you may find the information behind the links at right to be helpful, or maybe not?