Behind the Eight Ball or Behind the Times?
Remember that prepositions are relator words -- they show the relationship between nouns or pronouns in space and time. When examining any prepositional usage, it is important to know which two nouns are being related through a preposition and then if the relationship is literal or figurative, spatial or temporal.
As a preposition that shows the spatial relationship between two things, behind is expressed by detrás de in the sense of something being in back of something else:
Mi mochila está detrás de la silla (My backpack is behind the chair).
El gato se metió detrás del estante de libros (The cat went behind the bookcase).
The opposite of detrás de is delante de (in front of):
Mi madre me espera delante de la casa (My mother is waiting for me in front of the house).
Another prepositional solution for behind is tras, which means following after (think of the Pied Piper):
Los perros iban tras los gatos (The dogs followed after the cats).
Adverbially, that is, modifying a verb to show manner, the word for behind is atrás. In this adverbial situation it means backwards: Note that it does not depend on the position of one thing or person with respect to another:
El soldado dio un paso hacia atrás (The soldier took one step backwards).
The opposite of atrás is adelante (forwards).
Finally, atrasado is the adjectival form of behind, hence atrasada or plural forms too, are possible. It means that something or someone is behind, as in not as fast as other similar people or things, such as a clock or watch or the progress of a project:
La construcción va un poco atrasada por falta de fondos (The construction is a little behind due to lack of funds).
Mi reloj está atrasado (My watch is behind/slow).