Quick answer
Посу́да is a single feminine noun meaning all the dishes together: мыть посу́ду — to do the washing up.
Pots and pans
On the table
Tools
In the kitchen
Посу́да is singular
Even for a whole sinkful: посу́да гря́зная — the dishes are dirty, with singular agreement. There is no plural, and посу́ды is the genitive, not “dishes”.
Стака́н or ча́шка?
Стака́н is a straight glass, ча́шка a cup with a handle, кру́жка a bigger mug, рю́мка a small glass for spirits, бока́л a wine glass. Russian is precise here and the words are not interchangeable.
Typical mistakes
посу́ды гря́зные
посу́да гря́зная
A collective singular noun.
Я мо́ю таре́лки — when you mean all of it
Я мо́ю посу́ду.
посу́да covers the whole lot.
Check yourself
What does посу́да mean?