DIRECT OBJECT PRONOUNS
These forms are used as the direct objects of verbs.

me: me
te: you (familiar)
lo: him, you (formal), it  (masculine, singular)
la: her, you (formal), it  (feminine, singular)
los: them, you all (masculine, plural)
las: them, you all  (all feminine, plural)
nos: us
os:  you all (familiar, plural, Spain)
se: himself / herself / yourself (formal) / themselves / yourselves (formal)

'lo and 'la' are used as the direct object forms of 'usted'. 'los' and 'las' are used for ustedes. A prepositional phrase (a l, a ella, a usted) is sometimes be added for clarity or for emphasis, primarily in spoken Spanish. 

Examples:
Me oyes?:  Do you hear me?
Te necesito.:  I need you.
Lo conozco.: I know him/I know you/I am familiar with it. (unclear)
Lo conozco a l.:  I know him. ('a l' makes it clear)
Lo conozco a usted.: I know you.
Yo me miro en el espejo.:  I look at myself in the mirror.
Ustedes se matan.: You are killing yourselves.

Note that these direct object prepositions cannot be used if the verb is only IMPLIED. In that case, the prepositional phrase forms are required: a  l, a ella, etc. 

Example:
A quin vio Ud.? A ella?:   Who did you see?  Her? (verb implied)
