A word mark is a type of trademark protecting a word itself. It is usually used to trademark brand names or slogans.
A word mark doesn't protect the graphical depiction of the text. This means that after registering a word mark, you'll be able to use it in a variety of fonts, colours and contexts, but the downside is that since the protection extends only the word, should anyone copy your standard styling but apply it to a different word, that wouldn't be considered trademark infringement.
Another implication is that the distinctiveness of a word mark can only result from the word itself. Therefore, a word mark might have a higher chance of refusal than other trademark types. The reason is that a brand name is more likely to be descriptive of the products/services the brand offers or similar to other already registered word marks. With a figurative trademark, there are naturally more options to distinguish the mark, both on objective grounds and from others.
Although it may seem like a word mark would be less desirable than a figurative trademark, the choice is not that clear-cut, and a word mark offers its own pros. It provides a broader level of protection compared to a figurative mark, which can only be used exactly as registered. Also, a name is a more identifying element for a brand than its logo, and something consumers are more likely to remember.
Note: In graphic design, a word mark (or a wordmark or a logotype) refers to a distinct text-only typographic treatment of the brand name. This treatment DOES include the font and other stylistic elements. Google's logo is a good example of what is considered a word mark. Bear in mind that these terms have different meanings within their fields. As explained above, in the IP law, a word mark would only protect the name 'Google' itself.