お疲れさまでした。
Otsukare-sama deshita.
Good work.

This phrase, one of the most commonly heard in the Japanese business world, conveys appreciative recognition of another person’s labors. It could be used, for example, to greet a colleague returning to the office from an excursion to a client’s factory in an outlying area; a more elaborate translation might be “Your hard work is appreciated.” The figurative intention is to assuage the other person’s fatigue and commend his exertions on the firm’s behalf. A similar expression is:

ご苦労さまでした。
Gokurou-sama deshita.
Well done.

This one, however, is generally reserved for use by higher-ups addressing the people who serve under them and by older employees addressing their juniors. Strictly speaking, it would be a breach of etiquette for a younger employee or an underling to say gokurou sama deshita to a senior colleague or boss—the one to use is otsukare-sama deshita. In less formal circumstances, the final word is often dropped from these expressions, yielding the abbreviated forms.

お疲れさま。
Otsukare-sama.
Good work.

and

ご苦労さま。
Gokurou-sama.
Well done.

The phrase otsukare-sama deshita is often employed as a form of farewell to a colleague or boss at the end of the work day. The following exchange between fellow employees features an everyday example of this sort of usage:

Sato-san:
お先に失礼いたします。
Osaki ni shitsurei itashimasu.
Pardon me, but I’m off.

Kimura-san: 
お疲れさまでした。
Otsukare-sama deshita.
Good work today.

This sort of comradely send-off can help alleviate the stiffness that dominates the atmosphere in some offices.

Grammatically speaking, otsukare-sama deshita is a past-tense phrase. The present-tense form, otsukare-sama desu, is also used, but under different conditions—when the work in question is ongoing. Let’s say a colleague, Mr. Horiguchi, is phoning in from outside the office (he’s about to call on the printing firm Dai-ichi Insatsu) to pick up his messages:

堀口です。いま第一印刷に向かっていますが、何か連絡は入っていますか。
Horiguchi desu. Ima Dai-ichi Insatsu ni mukatte imasu ga, nani ka renraku wa haitte imasu ka.
This is Mr. Horiguchi. I’m on my way over to Dai-ichi Insatsu. Are there any messages for me?

You reply:

お疲れさまです。少々お待ちください。
Otsukare-sama desu. Shoushou omachi kudasai.
Good work. Just a moment, please. I’ll check.

– Source: A handbook of common Japanese phrases –