と details
と — joins nouns into a complete list, marks who you do it with, and closes a quotation
パンと牛乳とバターを買ってきてください。
pan to gyuunyuu to bataa o katte kite kudasai.
Please pick up bread, milk and butter. neutral
昨日、同僚と食事に行きました。
kinou, douryou to shokuji ni ikimashita.
I went out to eat with a colleague yesterday. neutral
Examples
and — と joins nouns into one closed list, with nothing left out
パンと牛乳とバターを買ってきてください。
pan to gyuunyuu to bataa o katte kite kudasai.
Please pick up bread, milk and butter. neutral
財布と鍵、持った？
saifu to kagi, motta?
Got your wallet and keys? casual
with — the person or thing the action is done together with
昨日、同僚と食事に行きました。
kinou, douryou to shokuji ni ikimashita.
I went out to eat with a colleague yesterday. neutral
来月、先方と契約について協議いたします。
raigetsu, senpou to keiyaku ni tsuite kyougi itashimasu.
Next month we will discuss the contract with the other party. formal
quotation — と closes what was said, thought or called, and hands it to the verb after it
田中さんは来られないと言っていました。
tanaka-san wa korarenai to itte imashita.
Tanaka said he wouldn't be able to come. neutral
明日は雨だと思うよ。
ashita wa ame da to omou yo.
I reckon it'll rain tomorrow. casual
Formation
{N}とattaches to noun普通形＋と（引用：明日行くと言いました・危ないと思う）attaches to plain clause
Notes
instead, because と after a predicate is read as the conditional —
行くと食べる says "whenever you go, you eat", not "goes and eats".
Related patterns
Similar
と closes the list and claims it is complete — パンと牛乳 says those two and nothing else — while とか offers samples of a longer list and skews spoken, so a read-out shopping list takes と and an off-hand "bread and milk and that" takes とか.
って is the spoken stand-in for this quotative と, freely dropping the だ that と keeps (雨だと言った／雨だって言ってた) — と is what writing and reported speech use, so a printed article never reaches for って.
と claims the list is complete and や claims it is not — パンと牛乳を買った says those two things were bought and nothing else, パンや牛乳を買った says those are two of the things, so a receipt read out takes と and a vague account takes や.
Lookalikes
One particle, three parses — after a NOUN と lists or accompanies (妹と行く); after a plain-form predicate what FOLLOWS decides, a verb of saying or thinking taking it as a quote (行くと言った) and anything else as the automatic conditional (押すと開く).
Combines with
という is this quotative と welded to 言う, and the weld has bleached the saying out of it — 田中という人 names someone nobody has quoted.
と思う takes the same quotative と, which is why the clause before it must end in a plain form and keeps its own だ — 雨だと思う; the bare 雨と思う belongs to written report style (原因は漏電と思われる), not to speech.
と同じ, と違う, と会う and と似ている take a fourth と that is neither list nor company but a required second party — 去年と同じだ is not "the same together with last year", and dropping the と leaves no sentence; 会う and 似る fill the same slot with に too.