If you’re bringing just your phone, odds are good that your hotel will be able to lend you a single plug adaptor for US->EU conversion; however, if you’re bringing more than one device, or if you’re not coming from Europe or the US, you should bring some adapters. Also, there is a difference between plug adaptors and power converters. Laptops and cellphones (and any other devices that are USB charged or have their own external transformer) should be fine, but other devices, such as medical equipment, may need a power converter.
The Czech Republic uses Type C and Type E power outlets, which are compatible with Type F, so if you need adaptors, you’re fine with anything that goes from your native plug type to either E C or F:
If you’re traveling beyond Czechia, these same adaptors will work in all of Europe and Russia, except for the UK, Ireland, and Malta.
Like almost all of Europe, the Czech Republic is on 230V power at 50Hz. Most small devices (e.g., cell phones, DSLR cameras) are charged via USB today, and it would be surprising if your normal USB plug didn’t work here. Equally, most laptops should be fine, as should any device that has a transformer built into the power cord:
If you’re bringing anything else, though, you should double check its power rating. The most common culprit for blowing up when traveling abroad is your hair dryer, but if you’re bringing your favorite coffee grinder, your beard trimmer, or some medical equipment, they’re equally likely not to work, and you should buy a transformer. If you do determine that you need to bring a transformer, the most important thing is the total wattage. Make sure the power rating for the transformer (in watts) exceeds the wattage requirement of all the devices you plan to plug in at the same time, added together.