Informal language is mainly used between people who know each other well, or in relaxed and unofficial contexts.
Here are some examples of informal English words and their standard equivalents:
clobber, gear, threads
As a rule, it's best to avoid informal vocabulary in most standard written contexts: it wouldn't be a good idea to use it in job applications, for example, or in a report:
✗ They laid into the government for failing to allocate enough funding
✓ They criticized the government for failing to allocate enough funding
Informal language also includes taboo vocabulary, often labelled as vulgar slang in dictionaries or thesauruses. This typically refers to sex or bodily functions and, since so many people are likely to find it offensive or shocking, it should generally be avoided.