Firstly, if you don't know the difference, it's very important that you learn to differentiate between the cows, bullocks (!) and bulls because in normal circumstances only bulls pose a real threat.
Simply....cows have udders with four tits!!! you will see these no problem and cows form no threat!
Bullocks don't have anything (their bal*s have been cut off when they were young because the farmer didn't want a bull

) Bullocks are much bolder than cows and will follow you ....very closely at times but will not harm you unless you make them jumpy i.e. behave like an idot or like you are scared...even then just think of them as being very very curious. Ignore the slabbers and heavy breathing!!
Bulls (you'll notice large dangling bal*s...sound familiar??

) and need serious respect, especially in the autumn when they are put in the field to do the business, if you get my drift. Personally, I wouldn't enter a field with a bull in it.....some are docile, even pet like, but others can be blo*dy dangerous, and you don't know what sort they are until it's too late. Most are untrustworth at the best of times, depending on the breed...Freisians (can't spell that) can be total bast**ds....the black and white ones. Another thing, a bull can be in a field with say twenty cows or more, so check carefully.
Lastly.....and very importantly....don't be an idiot and take a dog with you....all of the above don't like that and you are placing yourself in real danger....cows will surround you and appear aggressive, bullocks will do the same only more so and may well attack and bulls may just kill it or you or both..they don't care. As someone rightly said earlier, most deaths are due to the presence of a dog.
Went on longer that I expected, but I hope this helps......I lived in the countryside most of my life and know the form, and I recognise it isn't easy for a town dweller to have any knowledge of this...take care .....