Symbols

Tarot relies heavily on symbolism.
Esoteric decks, such as Thoth and to a lessor extent, the Waite-Smith make use of every symbol on every card. This is why I always recommend you purchase the book written specifically for your deck if one is available. It will usually explain the designer's use of symbolism in the deck. A dictionary of symbols is also an excellent tool for discerning deeper meanings in the cards. It is especially useful for decks which have no separate book available.

For an example where having the book enhances the interpretation, let's look at the six of wands in "The Haindl" The card shows six spears of equal height on a background of ivy leaves, somewhat suggestive of a wreath. If you look in the booklet that comes with the deck you find " The six spears suggest cooperation, the vines signify victory." but if you look in Rachel Pollack's book "The Haindl Tarot: Volume Two" you read that "the background shows a detail from a picture 'Dionysus'. Dionysus was a god of ecstacy leading his followers in the ancient dance..." This gives you further insight into what the designer was trying to convey.

Using the Waite-Smith deck, let's examine the same card.
We see a man on a horse carrying a staff with a laurel wreath on top. If we look up laurel in a dictionary of symbols we find "Triumph, victory", so even if did not have Waite's book we could get an idea of what the card meant from the symbolism augmented by the picture, which also suggests a triumphant march.
If we were not sure of the type of wreath, and looked up the word "wreath", we find "..glory, victory, supremacy..." which still conveys the meaning. If you look in the bibliography of most modern Tarot books, you will probably find at least one dictionary of symbols listed. They are especially useful for understanding cards that you have difficulty with.