I would say, in Japanese, the postposition/particle「と」"to" has a meaning similar to "and" in English, while the phrase「~と 一緒に」"~to issho-ni" has the meaning of "together with..." as mentioned by the above respondent. There is also another postposition/particle named「や」"ya", and it means also "and" in English. However, if I recall it correctly, when「と」is used for expressing a number of things, it is required to list out all of them; while for「や」, there is no such restriction. For instance, 1) 貴方は 明日 私と一緒に 図書館に 行きませんか。= Would you like to go with me together to library tomorrow? 2) 私は りんごと 苺と オレンジが 好きです。= I like apple, strawberry and orange. (i.e., only the mentioned three things are my favorite fruit.) 3) 私は スーパーから 沢山の物を買った：野菜や 牛肉や 果物や 等だった。= I bought a lot of things from the supermarket: vegetable, beef, fruit, etc. (i.e., I bought a lot of things, but I could only list out the mentioned ones.) I am not very sure about the structural correctness in e.g. 3), but at least you may have an idea the sort of differences between「と」and「や」. N.B. 「と」also has a function of "that" to link a subordinate clause into the main clause. For instance, (私は) 君が/貴方が とても 美しいんだと 思います。= I think that you are very beautiful. (This could be a good sentence if you are about to chase some look-gooding Japanese chicks, or any ladies that understand Japanese in general :)

I am not really well versed with particles but in my experience isshoni would most often be translated as together. Like in lets go all together for example. So you could drop the subjects. Isshoni taberu for example. "To" would require to name some Person. But aside from this kind of writing style related thing I would also like to know if there are any other differences.

'Issyo ni' means together as said above, and 'to' is used when linking items or people etc together. It is kind of difficult to explain how to use them, I think the best way is to study lots of example sentences and listen to Japanese natives speaking as much as possible.　In 'to issyo ni' in the other comment the 'to' is not connected to 'issyo ni', it is attached to the person/thing/word before it imo. I'll try and think of some examples anyway. boku ha bob to issyo ni gakkou ni itta. boku to bob to jane ha issyo ni gakkou ni itta. (there is no 'to' before isyo ni here). - person a: kyou eiga wo mi ni itta. (I went to saw a movie today) person b: hitori de? kanojyo ha? (by yourself? what about your girlfriend?) person a: issyo ni itta. (we went together) kotomotachi ga issyo ni uttateru The above post says that 'ya' also means 'and' in English but I think it's a bit different. Also when people are talking most people use 'toka' instead of 'ya'. Hope this helps.