Translation:What time is our Chinese class?
You can probably get away with treating them as functionally equivalent. There may be some technical differences, but commentators disagree as to what they are. Also, there are regional preferences with respect to these terms and several other similar ones:
It's said to be a sound-meaning (also called "phono-semantic") compound, with "言" being semantic and "吾" being phonetic. However, the phonetic parts of many characters are more relevant to old or middle Chinese.
"吾" itself is also said to be phono-semantic, with "五" contributing the phonetic part, so there's some layering and arbitrariness, which is common.
In any event, tracing it back, it seems that "五" is a phonetic inclusion, and doesn't make a semantic contribution.
Wiktionary often has this sort of info.