The core use of google photos is primarily centered around backing up your photos and videos. When you first setup the app, it will ask you a few introductory questions. Specifically how high of a resolution do you want to save your photos at and if you want to do it over wifi only or anytime your phone is connected to the internet.
Let's talk size. Specifically, file size. Google gives you two options: High Quality and Original. For a detailed breakdown of the difference in sizes see this blog post at the Huffington Post
. Google gives you unlimited image uploads if you choose high quality and the images are downsized to 16 megapixels which is what most new phones today are shooting anyway. This is the option I recommend unless you really really insist on having the originals take up space on google photos and you eventually plan on paying for storage. Google gives you 15 gigabytes for free by default and if you choose the high-quality option your photos won't go against that 15 gig limit.
If you just want to feel secure at night that your photos are safe, and don't want to pay for it, choose the High-quality option. In regards to the second question about uploading with cellular or wifi, that is completely up to you. When traveling, it's unlikely you will have an unlimited internet connection. I highly recommend you choose wifi (you can always change this later in the settings).
T-mobile has been a huge help in traveling abroad!
I recently got an unlimited data package with T-Mobile so I just switched from wifi to cellular (at least for my photos, my videos still require wifi since they are so big). The video is compressed a little bit, and I believe it does go against your 15 gig limit, so keep that in mind.
Backing up your photos on the fly works great in the US, but the best part is that even overseas I am able to use T-mobile just like I was back home (Data and Text). This takes a lot of fear away because, with my phone, I can google away almost any problem I run into, and I can back up my photos whenever I want, not just at the hotel.
The iPhone App does not backup your photos automatically, you need to open the app occasionally to back it up
Now let's talk about Android and IOS differences. Android allows Google to constantly work in the background. What does this mean for you? Well if you have android you never really need to open the app unless you want to look at your photos. iOS, however, does not allow the app to work in the background so to backup your photos you need to open the app and leave it running while it backs up.