Both like and such as are commonly used to compare two (or more) things. However, if you wish to write clearly and unambiguously, it's important to understand the distinction between the two.
When used to compare two or more things, the word like essentially means "similar to" or "in the manner of." In contrast, such as essentially means "for example."
Therefore, the following two sentences have different meanings:
Carnivores like wolves generally have large, sharp teeth.
Carnivores such as wolves generally have large, sharp teeth.
The first means that most carnivores that are similar to wolves have large, sharp teeth. It implies that carnivores that are not similar to wolves--say, for example, sharks--do not have large, sharp teeth.
The second sentence tells us that a wolf is an example of a carnivore, and that most carnivores--including wolves, and including sharks--have large, sharp teeth.
The insertion of a comma can affect meaning, too. A "like XXX" phrase enclosed in commas generally means you're making an explicit match or link between two things. In essence, you are equating the two. For example:
Seals, like dolphins, are mammals.
This sentence is using dolphins as a referent and stating that seals are similar to dolphins in the specific way described in the sentence (i.e., they are both mammals). In a sentence of this form, the noun after like is generally assumed to be more familiar to the reader than is the subject of the sentence. (I.e., this sentence suggests that the writer expects the reader to be more familiar with dolphins than with seals.) This sentence is correct as written. Replacing like with such as in this case would make the sentence incorrect, because a dolphin is not a type of seal. If we wanted to use such as in this sentence, we'd have to change dolphins to a type of seal (e.g., "Seals, such as the Weddell seal, are mammals.").
Unless you want to make that explicit link between the two things, you generally want a "such as XXX" phrase inside your commas. For example:
Some actors, like Jonathan Frakes, are quite tall.
There's nothing really wrong with this sentence, if you are trying to get across that Jonathan Frakes is quite tall, and that some actors resemble him in being quite tall. But if you were trying to give Jonathan Frakes as an example of a tall actor, rather than as the definition of a tall person, then you would use such as:
Some actors, such as Jonathan Frakes, are quite tall.
(This sentence would also work with including instead of such as, but I won't go there right now.)
9 years ago