There’s no subway service at LGA. You can take a city bus to get to the subway and/or into Manhattan, including the 7, but check to make sure what track work is going on. The 7 sometimes has limited service on weekends. There are also a couple of private bus/van services from LGA into the city. The names are escaping me right now. You might be better off just taking an Uber or a cab, depending on how many of you there are.
They are just two different lines. Why are you asking? If you are worried about service changes (and the NY subways have been shit b/c they are fiscally strapped and there is constant weekend work), look at this site: www.mta.info | The Weekender. Day of. .. just click on any station and it will tell you if there are any service changes.
On the 7 as we speak. Thanks for all the advice as we just had an amazing day at The Open. It was brutally hot but well worth it to see my wife smiling all day as we watched Djoka win in 4 on Ashe. Happy 50th Katie!!!