Related thread: Basic Economy Airport and Plane Experiences (First or Second Hand)

If purchased on united.com you will have to accept a warning about choosing Basic Economy and there will be orange "Basic Economy" highlights everywhere during the booking process.

Link to UA's description of how these fares will work: Basic Economy.

Here are the key facts:

• No free seat assignments until check-in. Seats will be assigned by the system. Flyers (including elite) have the option to purchase regular economy and preferred economy (white seats with black triangle on seat map) seats after ticket purchase. E+ seats are not available for purchase until check-in. If you don’t purchase an advance seat assignment, your seat will be automatically assigned to you prior to boarding, and you won't be able to change your seat once it's been assigned.
• No guarantee of adjacent seats with companions
• No voluntary ticket changes after 24 hour purchase period including SDC (occasionally 1K's have successfully used Same Day Standby and received a confirmed seat without penalty)
• Carry on limited to one personal item unless the customer is a MP Premier member, primary cardmember of a qualifying MileagePlus credit card, or Star Alliance *G, or traveling on an international BE product
• Customers ineligible for carry-on who bring one to the gate will be charged a $25 convenience fee to gate-check in addition to standard baggage fees (source: @united twitter)
• Customers are not eligible for premium cabin upgrades either via CPU, mileage upgrade award or PlusPoints
• Customers will board in the last boarding group (currently Group 5) unless the customer is a MP Premier member, primary cardmember of a qualifying MileagePlus credit card, or Star Alliance *G, or traveling on an international BE product
• Companions on same PNR will have same boarding group and carry-on if one on the PNR has a waiver
• No combinability with regular economy fares or partner carriers. Interline travel is not permitted.
• Normal PQPs / RDMs based on fare and elite multipliers
• No PQFs
• Flight(s) do count towards the 4 flight minimum for UA-operated
• Full Lifetime mileage
• Domestic Basic Economy tickets will use booking code 'N'. International BE can be in any class.
• Online check-in only with paid checked bag, otherwise need to see a United representative to verify the onboard bag allowance and receive a boarding pass
• Canceling a basic economy ticket will receive a flight credit for the price of the ticket, minus a fee. For domestic flights, the fee is $49.50 for one-way tickets and $99 for roundtrip tickets. For international flights, the fee is $99.50 for one-way tickets and $199 for roundtrip tickets

In air, passengers will receive the same standard economy inflight amenities including United Economy dining options, inflight entertainment, United Wi-Fi (availability depending on the flight)

To avoid domestic BE fares ITA Matrix ~bc=N . Ignoring them on international flights is much trickier because they do not have a consistent pattern in the fare basis; it appears to change by market / region. (That said, as UA is lifting BE from legacy GDS systems, it may result in them disappearing from ITA Matrix anyway).

Google Flights -- select 1 Carry-on bag

NO UNUSED TICKET MAY BE USED TOWARD THE PURCHASE
OF A BASIC ECONOMY TICKET.
-------
NO RESIDUAL VALUE MAY BE USED TOWARD THE PURCHASE
OF A BASIC ECONOMY TICKET.

How is this different from buying up BE to regular economy?

related threads
New UA/*A TATL -LGT Economy fare - no free first bag, no changes/upgrades allowed

Benefit impact of restricted economy fares on UA Elites (Basic Econ, -LGT, Light Econ

Pre-announcement speculation thread (now closed) New "Budget Economy" fares

United's Basic Economy - Discussion, Q&A, ... {Archive}