As http://www.vault.com/blog/admit-one-vaults-mba-law-school-and-college-blog/fine-tune-your-grammar-likely-like-vs-such-as-vs-asas-ifas-though says:
Use like before a noun or pronoun. Use as before a clause, adverb or
prepositional phrase. Use as if and as though before a clause. Like is
generally used as a preposition in such a context. As is generally
used as a conjunction of manner while sometimes serving as a
preposition with the meaning of "in the capacity of". As you can tell,
the focus of the comparison shifts from the noun when used with like
to the verb when used with as, as if, or as though.
- My mother's cheesecake tastes like glue.
- I love frozen pizza because there is no other snack like it.
- My mother's cheesecake tastes great, as a mother's cheesecake should.
- There are times, as now, that learning grammar becomes important.
- He golfed well again, as in the tournament last year.
- He served as captain in the navy.
- He often told half-truths, as any politician would.
- He looks as if he knows me.
- It looked as if a storm were on the way.
- He yelled at me as though it were my fault.
- He behaves like a child.
- He behaves as if he were a child.
- Like his brother, he plays chess.
- As I told you, they have well-mannered children.
- He works as a window cleaner.
- As President, he was very popular.
- As I was playing football, I met my best friend.
- I love sciences, like physics.
Some animals, such as lions can be dangerous.
As she's contagious, she won't go to school.
In sum, on the GMAT, use like before a noun or pronoun when
emphasizing similar characteristics between two persons, groups or
things. Use such as before a noun or phrase when introducing examples.
- Correct: I enjoy playing musical instruments such as piano and violin.
- Wrong: I enjoy playing musical instruments like piano and violin.
In your examples you should use either "such as" to introducing examples of objects or "like" to show similar characteristics and similarity.
- I love big dogs such as a German Shepherd and a Labrador. - means you love a German Shepherd and a Labrador.
- I love big dogs like a German Shepherd and a Labrador. - means you love big dogs that resemble a German Shepherd and a Labrador altogether.
Notice the usage of the conjunction "and" and also notice the
comma that isn't at all necessary there.
- I can play quite a few musical instruments such as the flute, the guitar, and the piano.
- I can play quite a few musical instruments like the flute, the guitar, and the piano.
The difference is identical to that mentioned above.