Two promises, made at two different times. The statutory sentence does two separate things. "Fully warrant the title to said land" is a statement about how matters stand when the deed is delivered: the grantor owns what is being conveyed, has the right to convey it, and is not handing over something encumbered beyond what the deal contemplates. "Will defend the same against the lawful claims of all persons whomsoever" is a forward-looking undertaking: if someone later turns up with a superior claim, the grantor will stand behind the grantee. A buyer reading a deed should separate the two, because a defect that existed on the day of the sale and a claim that surfaces ten years later are different events, and the second is the one the promise to defend is written for.
General versus special is the distinction that costs money, and one state codifies the difference in two adjacent sentences. Virginia gives statutory effect to each phrase. Under section 55.1-354 of its code, a covenant that the grantor "will warrant generally the property hereby conveyed" binds the grantor and their heirs and personal representatives to "forever warrant and defend such property unto the grantee and his heirs, personal representatives, and assigns against the claims and demands of all persons". Section 55.1-355 gives the narrower phrase, that the grantor "will warrant specially the property hereby conveyed", the same effect except in its last words, which limit the defense to claims "of the grantor and all persons claiming or to claim by, through, or under him". The general form answers for everyone; the special form answers only for the grantor and those claiming through the grantor. That is the whole distinction, and it decides who pays in the ordinary case: a claim that traces back to an owner three transfers ago does not come through the grantor, so a special warranty, also called a limited warranty, leaves it with the grantee. Institutional sellers that never lived in a property, such as a lender disposing of a foreclosed house or a court-appointed fiduciary, generally offer the narrower form for the obvious reason that they know nothing about the history.
State practice varies, and two named examples show how much. Florida prescribes a statutory warranty-deed form and gives it full common-law covenants. California's ordinary conveyance is a grant deed, whose form is set out at Civil Code section 1092 and whose covenants are implied by section 1113 from the single word "grant". Those implied covenants are considerably narrower than a full warranty, and published guidance on encumbrances on this site works through exactly why. The practical point for a buyer is that "what does the deed in my state promise" is a real question with a local answer, not a formality.
The limit that decides how much the promise is worth. A covenant of title is a contractual claim against a person. Collecting on it requires that the person or their estate can be found, has assets, and can be sued within whatever limitation period the state applies. An out-of-state relocation, a dissolved company, an estate long since distributed and closed, or simple insolvency each reduces the covenant to a right on paper. That is the reason a warranty deed and a title policy are not alternatives: one gives a claim against a person, the other gives a claim against an insurer, and buyers in an ordinary financed purchase end up with both.