Verbal means “related to words”; a written agreement is just as verbal as an oral one. People increasingly use it as the opposite of “written,” but there's still a band of brave souls who resist it. If you mean something spoken, use oral. Samuel Goldwyn ignored this distinction in his quip, “A verbal agreement isn't worth the paper it's written on.”
A verb is a part of speech that shows action or a state of being. Some examples: talk, worry, demonstrate, exist, calibrate, and be.
Note that English is flexible enough to allow many words or phrases to operate as verbs, though most of the new coinages are still considered informal, and a lot of them won't show up in most dictionaries. The noun pants, for instance, can become a verb meaning “to pull someone's pants down” (usually as a prank); to gift (meaning “to give a gift”) was popular some years ago (and became fashionable again when Seinfeld coined the verb to re-gift); businessfolk are fond of verbs like to transition to to liaise. I find most of the businessy ones ugly as sin, but they don't violate any “rules.”
If you spot a verb “in the wild,” you can classify it according to a number of categories:
A comprehensive description of a verb would include something on all these categories. Here are the first two sentences of Moby-Dick, with more or less full descriptions of the main verbs (which I've italicized) in each clause:
Call me Ishmael.
Some years ago — never mind how long precisely — . . .
. . . having little or no money in my purse . . .
Having is the present participle of the active, indicative, transitive verb to have; the present participle indicates a continuing aspect.
. . . and nothing particular to interest me on shore . . .
The infinitive, usually (but not always) spotted by the accompanying particle to. To interest is an active and transitive verb.
. . . I thought . . .
First person, singular, past tense, active voice, intransitive, completed aspect, indicative mood.
. . . I would sail about a little and see the watery part of the world.
Here the verbs are would sail and [would] see: First person, singular, indicative, future tense, active voice, transitive, continuous aspect, subjunctive mood. (Whew!)
The only people who routinely have to break down verbs this obsessively are those learning a foreign language. And I've made no effort to define things precisely; linguists still wrangle over the finer points, and I'm simply not qualified to give the final word on anything. Still, this should offer you a basic understanding of what's going on, and give you the vocabulary to discuss verbs in more detail should you want to do that.
See Wasted Words.
Having a large vocabulary can never hurt, but you should use your energy wisely. Though knowing words like obnubilate, hebetic, and tergiversation can make you the envy of your crossword-puzzle-playing friends, in writing you'll get more mileage out of knowing the precise meaning of more common words. Can you distinguish climatic from climactic? — tortuous from torturous? — incredible from incredulous? — turgid from turbid? They're very different, but often confused. For a good guide, to these pairs and others, see Maxwell Nurnberg in the “Additional Reading” section.
Don't use obscure words just because you can; ostentation leads only to obfuscation. Using mirific where amazing or wonderful will do is just showing off and intimidating your audience. See also Long Words.
Voice is a technical term in grammar to describe a verb: the common voices in English are active and passive. Voice describes whether the subject of a sentence is acting or being acted upon. See Passive Voice for details. [Entry added 9 April 2001.]